Predictors and consequences of voice disorders in teachers: A systematic review

Abstract

Background

Voice disorders are among the most frequent occupational health problems in teachers, affecting vocal performance and quality of life. Following the PRISMA 2020 statement, this systematic review identifies the main predictors and consequences of voice disorders in teachers and provides an updated synthesis of evidence published between 1993 and 2023.

Methods

A systematic search was conducted in Web of Science, Scopus, and EBSCOhost using terms related to “voice disorders”, “teachers”, “risk factors”, and “consequences”. Studies were included if they addressed predictors and/or consequences of dysphonia caused by maladaptive vocal behaviors, used detailed empirical designs, and focused exclusively on teachers (from preschool to university levels, including preservice teachers). Only articles in English or Spanish were considered. Exclusion criteria comprised interventional studies, reviews, book chapters, and research on other populations. Data extraction and quality assessment were performed independently by two reviewers, and only studies rated as excellent quality (100%) were included to minimize bias.

Results

Twenty-one studies met the inclusion criteria after screening 108 records, with sample sizes ranging from 12 to 1980 teachers. Predictors included personal risk factors, vocal symptoms, and work-related risk factors subdivided into environmental occupational and organizational work factors. Acoustic parameters (fundamental frequency, jitter, shimmer, and signal-to-noise ratio) were analyzed as vocal indicators. Consequences were mainly absenteeism, presenteeism, and reduced quality of life, although heterogeneity and potential publication bias should be considered.

Conclusions

Voice disorders in teachers have a multifactorial origin, highlighting the need for preventive interventions addressing both individual and work-related factors.

Resumen

Contexto

Los trastornos de la voz constituyen uno de los principales problemas de salud laboral en el profesorado, afectando al rendimiento vocal y a la calidad de vida. Siguiendo las directrices PRISMA 2020, esta revisión sistemática identifica los principales predictores y consecuencias de los trastornos de la voz en docentes y ofrece una síntesis actualizada de la evidencia publicada entre 1993 y 2023.

Método

Se efectuó una búsqueda sistemática en Web of Science, Scopus y EBSCOhost con términos relacionados con “trastornos de la voz”, “docentes”, “factores de riesgo” y “consecuencias”. Se incluyeron estudios empíricos en inglés o español centrados exclusivamente en docentes (desde el nivel preescolar hasta el universitario, incluyendo docentes en formación), que abordaran predictores y/o consecuencias de disfonías causadas por comportamientos vocales desadaptativos. Se excluyeron estudios de intervención, revisiones, capítulos de libro y trabajos sobre otras poblaciones. La extracción y evaluación de la calidad se realizaron de forma independiente por dos revisores, incluyendo únicamente los estudios de calidad excelente (100%) para reducir el sesgo.

Resultados

Veintiún estudios cumplieron los criterios de inclusión tras examinar 108 registros, con muestras de entre 12 y 1.980 docentes. Los principales predictores fueron factores personales, síntomas vocales y factores laborales (ambientales y organizativos). Los parámetros acústicos se analizaron como indicadores vocales. Las consecuencias más frecuentes fueron el absentismo, el presentismo y la disminución de la calidad de vida.

Conclusiones

Los trastornos de la voz en docentes tienen un origen multifactorial, lo que resalta la necesidad de implementar intervenciones preventivas que aborden tanto los factores individuales como los relacionados con el entorno laboral.

Introduction

Voice disorders in teachers

Voice disorders are defined as alterations in pitch, loudness, vocal quality or flexibility that interfere with communication or reduce voice-related quality of life (Cobeta, Núñez, & Fernández, 2013; Roy et al., 2004). Teaching is a high-risk profession when it comes to developing voice disorders (Moreno, Calvache, & Cantor-Cutiva, 2022; Van Houtte, Claeys, Wuyts, & Van Lierde, 2011). In fact, the International Labor Organization (ILO) defines teaching as the first professional category at risk of contracting occupational voice diseases (López Espinosa, Álvarez Sintes, & Molina Linares, 2023). It is estimated that the prevalence of voice disorders in the general population is around 10%, while these values are around 40% among teachers, and can reach up to 80% (Cantor Cutiva, Vogel, & Burdorf, 2013; González-Gamboa et al., 2022). Additionally, 58% of teachers have reported voice problems at some point in their careers (Garcia Martins, Neves Pereira, Bosque Hidalgo, & Mendes Tavares, 2014). The main vocal disorders reported in this profession are functional dysphonias and vocal nodules, representing 55% and 22.5% of cases respectively (Agostini-Zampieron, Barlatey-Frontera, Barlatey-Frontera, & Arca-Fabre, 2013). As a result, vocal nodules have been officially recognized as an occupational disease in Spain since 2007 (Castejón Fernández, 2014).

Predictors of voice disorders

Contrary to common assumptions, most voice disorders do not occur suddenly, but there are several predictors capable of indicating their possible onset. These include symptoms such as vocal fatigue, hoarseness, straining when speaking, throat discomfort, difficulties singing or muscle tension (Garcia Martins et al., 2014; Kooijman et al., 2005). The most studied predictor has been vocal fatigue, which can be defined as the gradual perception of phonatory effort associated with decreased vocal performance, typically accompanied by voice-related symptoms that improve with vocal rest (Moreno et al., 2022). With the aim of identifying this sensation, Nanjundeswaran, Jacobson, Gartner-Schmidt, and Verdolini Abbott (2015) developed a self-perception questionnaire known as the Vocal Fatigue Index (VFI). Recently, Cantor Cutiva, Malebrán Bezerra de Mello, and Hunter (2020) translated and adapted this tool into Spanish, designated as the Índice de Fatiga Vocal (IFV).

In line with recent consensus efforts, terminology in the present review is informed by the framework proposed by Hunter et al. (2020), which distinguishes between vocal demands, vocal demand response (adaptive or maladaptive), vocal effort, and vocal fatigue, in order to improve conceptual clarity and consistency across studies.

Likewise, several risk factors play a fundamental role in these disorders, particularly those related to occupational voice use. Generally, voice disorders are related to the work environment, personal factors such as sociodemographic aspects and excessive vocal demands, as well as general health and mental health (Byeon, 2019). Specifically, da Rocha, de Lima Bach, do Amaral, Behlau, and de Mattos Souza (2017) found that teachers who had a common mental disorder such as anxiety or depression had twice the risk of developing a voice disorder. Ultimately, identifying robust predictors associated with the presence of voice disorders enables early detection and supports the development of appropriate prevention protocols.

Vocal indicators

In addition to perceptual and self-reported symptoms, several acoustic parameters have been analyzed as objective indicators of vocal function rather than predictors. Measures such as fundamental frequency (F0), jitter, shimmer, and signal-to-noise ratio (SNR) are useful for identifying early alterations in voice quality associated with vocal fatigue or maladaptive vocal demand responses. For instance, Vera García, Merino Gálvez, Prieto Merino, and Carazo Díaz (2022) observed significant deterioration in teachers’ acoustic parameters after the school day, especially in the second and third trimesters. Although specific cut-off values vary across studies, increases in jitter and shimmer or decreases in SNR typically indicate a reduction in vocal efficiency.

Consequences of voice disorders

Voice disorders not only impact the professional and personal lives of teachers, but also imply a significant financial burden on society (Van Houtte et al., 2011). Professionally, the main problems found in the literature are absenteeism and sick leave. Over their entire careers, approximately one-third of teachers were absent from work at least once due to voice problems (Mesquita de Medeiros, Ávila Assunção, & Barreto, 2012). Moreover, voice disorders are a common cause of sick leave, often extended over long periods, with an estimated average duration of 120 days (Matos de Souza, Campos Granjeiro, Patrícia de Castro, da Cunha Ibiapina, & Ferreira Oliveira, 2017).

From an economic standpoint, Cantor Cutiva and Burdorf (2015) conducted a study where it was calculated that, on average, the total costs of voice symptoms per month are 458 dollars. Of this total, 64% was attributed to reduced work productivity, 33% to voice-related absenteeism, and 3% to healthcare use.

It should be noted that the studies reporting economic costs and duration of sick leave were conducted in specific national contexts, mainly in countries such as Colombia and Brazil. Consequently, these figures must be interpreted within the framework of each country's healthcare system and labor regulations, as both economic impact and sick leave duration may vary substantially across countries.

On the other hand, previous literature has demonstrated the effectiveness of speech therapy interventions in relation to the pathologies discussed. Guzman et al. (2013) indicated that the use of phonatory tasks with straw exercises can produce immediate acoustic effects in dysphonic voices. On the other hand, Meier and Beushausen (2021) revealed that most teachers in their study implemented vocal training content in their daily work life, which led to an improvement in vocal ability and performance. Furthermore, Gassull, Godall, Polini, Amador, and Casanova (2020) observed significant improvements in the Voice Handicap Index (VHI), classroom voice management, perceptual and subjective parameters of the voice and some functional lesions after the application of the speech therapy intervention. Therefore, speech therapy has proven to be effective both in the short and long term for improving teachers’ voices and their quality of life (Mendes Ferreira et al., 2013).

Although previous research has documented the impact of teachers’ voice disorders on students’ learning outcomes and on the teacher–student relationship, these domains fall outside the scope of the present review. The present review focuses on consequences related to teachers’ vocal health and occupational functioning.

The present study

Due to the high incidence of voice disorders in teachers and the significant consequences they cause, as well as the manifestation of numerous predictors that warn of their appearance, this systematic review aims to identify key predictors for the early detection of voice disorders and examine their consequences within the teaching population.

Specifically, the review focuses on those predictors and consequences related to pathologies associated with maladaptive vocal demand responses, traditionally described as vocal misuse or abuse. That is, functional dysphonias such as Muscle Tension Dysphonia (MTD) or organic-functional dysphonias such as nodules and polyps will be considered.

Method

Search procedure

A systematic review was carried out based on the PRISMA methodology (Parums, 2021). The databases consulted were Web of Science (WOS), Scopus, and EBSCOhost. Within EBSCOhost, the following databases were accessed: APA PsycInfo, CINAHL Complete, and MEDLINE. No time restrictions were applied; therefore, the search covered publications from 1976 to 2024, yielding 293 results. Subsequently, a preliminary filter was applied based on document type and language, selecting 210 articles available in English and Spanish. The last search consulted was carried out on April 2, 2024.

The search strategy was identical in all databases and was compiled under four semantic categories called “topics”. The first topic refers to those terms related to voice problems, including dysphonia, voice disorders, vocal difficulties, nodules, polyps, etc. The second topic refers to terminology referring to teachers, such as teacher, educator, tutor, etc. The third topic refers to all those terms associated with the predictors of voice disorders, such as symptoms, risk factors, warning signs, etc. Finally, the fourth topic indicates those words related to the consequences of voice disorders, such as absenteeism, sick leave, short-term and long-term effects, etc. In each topic, the Boolean operators (*) and “OR” were used with the aim of broadening the search as much as possible. In addition, the integration of all modules was carried out through the Boolean operator “AND”. Table 1 shows the global search term. A total of 108 documents were included for the following screening procedure.

Table 1.

Search strategy.

#1: Voice problems	“voice disorder*” OR “voice problem*” OR dysphonia OR “vocal impairment” OR “voice difficult*” OR “vocal complaint*” OR “vocal dysfunction” OR “vocal effort” OR “functional voice disorder*” OR FSVD OR FVD OR “functional dysphonia” OR “hyperfunctional dysphonia” OR “hyperfunctional voice disorder*” OR “muscle tension voice disorder*” OR MTVD OR “muscle tension dysphonia” OR “vocal cord nodules” OR “vocal cord polyps”
#2: Teachers	teach* OR tutor OR schoolteacher OR educator OR scholar OR “faculty member” OR professor OR instructor OR “voice professional”) AND (predictor* OR symptom* OR prevent* OR “marker*” OR “risk factor*” OR “vocal risk factor*” OR “warning sign*” OR “alarm sign*” OR “at-risk” OR report* OR measur* OR indicator* OR diagnostic OR “vocal fatigue” OR sick OR “vocal hygiene” OR “voice-related outcome measure*”
#3: Predictors	predictor* OR symptom* OR prevent* OR “marker*” OR “risk factor*” OR “vocal risk factor*” OR “warning sign*” OR “alarm sign*” OR “at-risk” OR report* OR measur* OR indicator* OR diagnostic OR “vocal fatigue” OR sick OR “vocal hygiene” OR “voice-related outcome measure*”
#4: Consequences	consequenc* OR “vocal care” OR “voice-related absenteeism” OR sequela OR “quality life” OR “sick leave” OR “long-term consequences” OR “long-term effects” OR “short-term consequences” OR “short-term effects”
Total	(#1) AND (#2) AND (#3) AND (#4)

Eligibility criteria

The inclusion criteria were the following: (1) Research that addressed the predictors and/or consequences of dysphonia caused by maladaptive vocal demand responses (e.g.: MTD, nodules, polyps, etc.); (2) Articles of an empirical nature that described the sample in detail; (3) Studies with a population focused exclusively on teachers (preschool, primary, secondary, university teachers, preservice teachers – teacher training students engaged in supervised teaching practice – who are already exposed to relevant vocal demands during classroom activities, etc.); (4) Documents written in English and Spanish.

The exclusion criteria were the following: (1) Studies that included or evaluated speech therapy or vocal intervention programs, whether for dysphonia caused by maladaptive vocal demand responses or other etiologies (e.g., psychogenic dysphonia, laryngitis, vocal fold paralysis, etc.); (2) Literature reviews, book chapters and other types of review documents; (3) Research focused on other populations such as singers, announcers, actors, etc.; (4) Documents written in languages other than English or Spanish.

A preliminary filter was also applied by document type, including only peer-reviewed empirical research articles, and by language, selecting works written in English or Spanish.

Data collection process

The screening process was carried out through the Rayyan platform according to the inclusion and exclusion criteria mentioned (Johnson & Phillips, 2018). This procedure was distributed in three phases with the aim of improving the understanding of the inclusion and exclusion criteria by the reviewers in each block, and, therefore, increasing the interjudge reliability indices (Belur, Tompson, Thornton, & Simon, 2021).

During the data preparation phase, the 108 articles retrieved after the database search were organized into three groups of 36 studies each, ordered alphabetically by the first author's surname. This distribution facilitated a balanced and independent review process by the two evaluators.

Two reviewers selected the articles independently in each phase to ensure unbiased decisions. Subsequently, they discussed the observed discrepancies and reached an agreement regarding the inclusion or exclusion of the articles. This procedure was repeated in each of the three phases. In order to analyze the interrater reliability, the Interrater Reliability (IRR) was carried out through Cohen's Kappa (see Table 2). Finally, a total of 47 articles met the inclusion and exclusion criteria of the screening section.

Table 2.

Comparative IRR test scores and κ statistic.

Categories	First IRR		Second IRR		Third IRR
	N studies	Percent	N studies	Percent	N studies	Percent
Total items coded individually	36	100	36	100	36	100
Screening decision—include^a	41		23		32
Screening decision—exclude^a	31		49		40
Individual coding—agreement for all^b	27	75	29	80.56	32	88.89
Joint coding—agreement for all^c	9	25	7	19.44	4	11.11
Lack of agreement after discussion	0		0		0
Agreement after arbitration	NA	NA	NA	NA	NA	NA
κ statistic^d	.49		.57		.78

Note: IRR = Interrater Reliability/NA = Not Applicable.

a

Total number of screening decisions made by the two judges, both included and excluded.

b

Total number of agreements included and excluded by the two judges.

c

Total number of studies that required discussion regarding inclusion or exclusion.

d

κ statistic was calculated based upon the number of inclusions and exclusions after the initial screening decision and prior to reconciliation.

Following the interpretation of Landis and Koch (1977), phase 1 showed moderate reliability (k = .49) as did phase 2, although in this case it was very close to being considered substantial (k = .57). Likewise, phase 3 obtained substantial reliability, positioning itself very close to almost perfect (k = .78). All of this indicates that the eligibility criteria were increasingly precise as the screening phase progressed.

Studies presenting incomplete or insufficient methodological or sample information were excluded to ensure data integrity. Heterogeneity among studies was qualitatively assessed in terms of population characteristics, study design, and measurement instruments. To minimize the risk of bias, only studies rated as excellent quality (100%) according to the Critical Review Form – Quantitative Studies (Law, Stewart, Pollock, Bosch, & Westmorland, 1998) were included in the synthesis.

Finally, 47 articles were read in full-text in the eligibility phase. Ultimately, 21 articles were included in this review, excluding 26 documents for different reasons (e.g.: interventions, non-teaching population, language, etc.). The identification, screening, eligibility, and inclusion process is summarized in Fig. 1, which presents the PRISMA 2020 flow diagram.

Fig. 1.

PRISMA flow diagram.

Table 3 summarizes the full-text studies that met the initial criteria but were subsequently excluded, with detailed reasons for their exclusion.

Table 3.

Studies excluded after full-text review and reasons for exclusion.

Authors (year)	Title	Reason for exclusion
Alves Silverio et al. (2008)	Actions in vocal health: a proposal for improving the vocal profile of teachers	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Astolfi et al. (2003)	Classroom acoustic assessment: Analysis of subjective answers and measured indices	The document was not an empirical research article
Behlau et al. (2012)	Epidemiology of voice disorders in teachers and nonteachers in Brazil: Prevalence and adverse effects	Compared teachers and non-teachers, not exclusively focused on the teaching population
Bovo et al. (2013)	Voice amplification for primary school teachers with voice disorders: A randomized clinical trial	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Cantor Cutiva and Burdorf (2015)	Medical costs and productivity costs related to voice symptoms in Colombian teachers	Did not provide a detailed description of the sample*
Costa Santos et al. (2020)	Quality of life, health and work of elementary school teachers	The study did not primarily address dysphonia or voice disorders, focusing instead on teachers’ general health and occupational conditions
Dokoza (2020)	Results of voice self-assessment of the mainstream teachers and the teachers of students with special educational needs in elementary schools	The article was written in a language other than English or Spanish
Gassull et al. (2020)	Effects of a Voice Training Program on Acoustics, Vocal Use, and Perceptual Voice Parameters in Catalan Teachers	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Guzman et al. (2013)	Immediate acoustic effects of straw phonation exercises in subjects with dysphonic voices	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Jardim et al. (2007)	Work conditions, quality of life, and voice disorders in teachers	The article was written in a language other than English or Spanish
Kooijman et al. (2005)	Muscular tension and body posture in relation to voice handicap and voice quality in teachers with persistent voice complaints	Did not provide a detailed description of the sample*
Kooijman et al. (2006)	Risk Factors for Voice Problems in Teachers	Did not provide a detailed description of the sample*
Leppänen et al. (2010)	One-year follow-up study of self-evaluated effects of Voice Massage™, voice training, and voice hygiene lecture in female teachers	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Meier and Beushausen (2021)	Long-Term Effects of a Voice Training Program to Prevent Voice Disorders in Teachers	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Mendes Ferreira et al. (2013)	Analysis of aspects of quality of life in teachers’ voice after discharged: longitudinal study	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Muñoz López et al. (2017)	Effectiveness of a short voice training program for teachers: a preliminary study	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Nerrière et al. (2009)	Voice disorders and mental health in teachers: a cross-sectional nationwide study	Did not provide a detailed description of the sample*
Nusseck et al. (2021)	Long-Term Effects of a Voice Training Program for Teachers on Vocal and Mental Health	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Ohlsson et al. (2016)	Voice Disorders in Teacher Students—A Prospective Study and a Randomized Controlled Trial	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Roy et al. (2001)	An evaluation of the effects of two treatment approaches for teachers with voice disorders	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Roy el al. (2002)	Voice amplification versus vocal hygiene instruction for teachers with voice disorders: A treatment outcomes study	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Roy et al. (2003)	Three treatments for teachers with voice disorders: A randomized clinical trial	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Roy et al. (2005)	Voice disorders in the general population: Prevalence, risk factors, and occupational impact	Not exclusively focused on the teaching population
Scanferla et al. (2023)	The Usability and Acceptability of the mHealth “Health and Voice” for Promoting Teachers’ Vocal Health	Focused on the effects of vocal intervention rather than on the predictors or consequences of voice disorders
Schneider et al. (2006)	Effects of Vocal Constitution and Autonomic Stress-Related Reactivity on Vocal Endurance in Female Student Teachers	Did not provide a detailed description of the sample*The study did not include a sufficient or relevant review of the literature*
Smolander and Huttunen (2006)	Voice problems experienced by Finnish comprehensive school teachers and realization of occupational health care	Did not provide a detailed description of the sample*

Note: Articles marked with an asterisk (*) were excluded after methodological evaluation using the adapted Critical Review Form – Quantitative Studies (Law et al., 1998).

Quality evaluation

In order to evaluate the quality of the articles, an adaptation of the Critical Review Form – Quantitative Studies (CRF-QS) test by Law et al. (1998) was carried out. In it, the following aspects were evaluated: (1) Purpose of the study; (2) Literature; (3) Sample; (4) Results; (5) Conclusions; (6) Score; (7) Interpretation. It is worth mentioning that all those articles that did not provide a detailed description of the sample were discarded. Furthermore, the interpretation of the scores was based on Faber, Bustin, Oosterveld, Elferink-Gemser, and Nijhuis-Van der Sanden (2015), where the studies were classified into: (1) Low methodological quality, with a score ≤ 50%; (2) Good methodological quality, with a score between 51 and 75%; and (3) Excellent methodological quality, with a score > 75%. Only studies rated as excellent (100%) were included to minimize potential bias and ensure reliability. The results of this quality assessment are presented in the Results section.

Document coding

A total of 21 articles were manually coded. The documents were coded under the following characteristics: (1) Articles (authors and year); (2) Sample (participants and teaching level); (3) Instrument; (4) Predictor (type and significance); (5) Consequence (type and prevalence). The category ‘participants’ includes sociodemographic variables such as sex or location. Likewise, the term “significance” only refers to those predictors that are significantly associated with voice disorders through the statistical value p.

Results

After applying the inclusion and exclusion criteria, 21 articles were selected. Document Coding was carried out that collected the most relevant characteristics of each of the studies (Table 4). The articles covered a period from 1993 to 2023. Regarding the sample, the studies included from 12 to 1980 participants, all of them agreeing that the majority or all were women. In relation to the level of education, 17 of the studies focused on the primary education stage, followed by 12 studies with secondary education, 6 studies with early childhood education and 4 studies with higher education. It is worth mentioning that 12 studies considered more than one educational level simultaneously, and it is common to find articles about primary and secondary education teachers.

Table 4.

Main characteristics of the articles included in the systematic review.

Article	Sample		Instrument	Predictor		Consequence
Authors (year)	Participants	Teaching level		Type	Significance	Type	Prevalence
Ählander et al. (2011)	467 teachers in Sweden (72% women)	Secondary	Self-constructed questionnaire	Asthma	.021	Professional help	38%
				Hearing aid	.001	Sick leave	35%
				Classroom conditions	<.05
				Strong odors	<.05
				Voice symptoms	<.05
				Group size	.001
Alrahim et al. (2018)	187 teachers in Saudi Arabia (55.1% women)	Primary and Secondary	Questionnaire based on previous studies	Public school	.016	Absenteeism	45%
				Stress	.04
				Smoking	.016
				History of hoarseness	.0001
				Respiratory infections	.017
				Number of students	.038
				Gastroesophageal eflux (GER)	.04
Alshuhayb et al. (2022)	604 teachers from Saudi Arabia (62.1% women)	Primary and Secondary	Questionnaire based on previous studies	Female gender	.001	Absenteeism	32%
				Family history	.001	Professional help	5%
				Number of classes	.001	Vocal training	30%
				No smoking	.001
				Loud voice	.001
Alva et al. (2017)	105 teachers from India (84% women)	Primary and Secondary	Self-constructed questionnaire	Difficulty in low voice	.049	Social isolation	26%
				Vocal effort	<.001	Quality of life	15%
				Vocal fatigue	.018
				Female gender	.04
				Respiratory infections	.004
				Low spoken voice	.018
				Tense voice	.036
Cantor Cutiva and Burdorf (2014)	438 teachers from Colombia (76% women)	Superior	Questionnaire based on previous studies	Hearing impairment	<.05	Absenteeism	7%
				Dust	<.05	Professional help	25%
				Environmental noise	<.05	Loss of productivity	69%
da Rocha et al. (2017)	469 teachers from Brazil (91.3% women)	Primary and Secondary	Self-report questionnaire	Severity and duration of symptoms	<.05	Sick leave	16%
				Hearing symptoms	<.05
				Sensory symptoms	<.05
				Classes higher than 4th	.046
				Common mental disorder	.001
da Rocha and de Mattos Souza (2013)	575 teachers from Brazil (91.3% women)	Primary	Self-administered questionnaire	Number of students	.020	Sick leave	15%
				Common mental disorder	.000	Vocal rest	68%
Evitts et al. (2022)	141 teachers from Sweden (65% women)	Secondary and Superior	Questionnaire based on previous studies	Burning	<.05
				Headset	<.05
				Air quality	<.05
				Pain	<.05
				Dryness	<.05
				Strain	<.05
Jonsdottir et al. (2015)	243 teachers from Iceland (94.6% women)	Infant	Questionnaire based on previous studies	Stress	<.001	Sick leave	17%
				Lump in throat	.007
				Hoarseness	.016
				Dry throat	.040
				Weak voice	.029
			Environmental noise measurements	Environmental noise	<.05
Kyriakou et al. (2020)	449 teachers from Cyprus (94% women)	Infant and Primary	Questionnaire based on previous studies	Clear throat	.012	Absenteeism	39%
				Allergies	.029	Professional help	15%
				Short breaks	.004	Loss of productivity	41%
				Vocal discipline	.000
				Echo	.007
				Infant school	.015
				Stress and anxiety	.021
				Smoking	.020
				Smoking in the past	.006
				Shout	<.001
				Respiratory infections	.001
				Airplane and/or street noise	.011
				Noise inside the building	.042
				Noise inside the class	.000
				Electronic noise	.012
				Outside noise	.009
				Outside noise of children playing	.004
				Cough	<.001
				Loud voice	.000
Laukkanen et al. (2008)	79 female teachers from Finland	Primary	Acoustic recording in reading	Alpha	.000
				F0	.000
				SPL	.036
			Acoustic recording in reading aloud	Alpha	.000
				F0	.000
			Acoustic recording in [a:]	F0	.030
				Jitter	.025
				Shimmer	.018
				SPL	.033
			Vocal Fatigue Index	Throat fatigue	.000
Leão et al. (2015)	1879 New Zealand teachers (72.6% women)	Primary and Secondary	Self-constructed questionnaire	Alteration in vocal quality	<.001	Professional help	49%
				Difficulty in vocal projection	<.001	Short leaves	71%
				Age	.016	Long leaves	30%
				Vocal effort	<.001	Presenteeism	60%
				Female gender	<.001
				Sore throat	<.001
				Education level	.016
				Broken voice	<.001
Lee et al. (2018)	1617 teachers from Korea (61.3% women)	Primary and Secondary	Self-report questionnaire	Physical education	.003	Presenteeism	50%
				High school	.031
				Female gender	<.08
				Work hours	.034
				No smoking	<.01
				Classroom teacher	.024
Mesquita de Medeiros et al. (2012)	1980 female teachers from Brazil	Primary	Self-constructed questionnaire	Depression and/or anxiety	<.001	Absenteeism	3.35%
				History of absenteeism	<.001	Short leaves	33%
				Respiratory infections	<.001	Long leaves	25%
				Sleeping problems	<.001
				Relationship with colleagues	.038
				Relationship with parents	.018
				School noise	.012
				Classroom noise	.007
Rajasudhakar and Savithri (2010)	12 female teachers from India	Primary	Acoustic recordings in/a/	F0	<.001
				Jitter	<.001
				Shimmer	<.001
			Acoustic recordings in speaking/reading	F0	<.001
RoscellaInja (2016)	78 Malaysian teachers (84.6% female)	Primary	Self-constructed questionnaire	Drinking alcohol	.012	Professional help	40%
				Smoking	.012	Vocal training	19%
				Morning sessions	.049	Presenteeism	87%
						Speech therapy treatment	1%
Rute Sanabria et al. (2023)	29 teachers from Colombia (52% women)	Superior	Questionnaire based on previous studies	Female gender	.000
				Number of students	.000
				Voice symptoms	.000
Sapir et al. (1993)	307 US teachers (84% women)	Infant, Primary and Secondary	Questionnaire based on previous studies	Clearing throat	<.0001	Absenteeism	32%
				Shortness of breath	<.0001	Professional help	21%
				Burning	<.0001	Labor impact	36%
				Difficulty in vocal control	<.0001	Speech therapy treatment	3%
				Sore throat	<.0001
				Effort when speaking	<.0001
				Stress and frustration	<.0001
				Vocal fatigue	<.0001
				Vocal discomfort	<.0001
				Number of years worked	<.05
				Throat tightness	<.0001
				Itching	<.0001
				Short breath	<.0001
				Hoarseness	<.0001
				Dryness	<.0001
				Weaker voice	<.001
				Lower voice	<.0001
				Less pleasant voice	<.0001
Van Houtte et al. (2011)	994 teachers from Belgium (67.4% women)	Infant, Primary and Secondary	Questionnaire based on previous studies	Primary school	.014	Absenteeism	38%
				Female gender	.003	Professional help	50%
				Loss of vocal range	.002	Short leaves	75%
				Vocal loss	.002	Long leaves	16%
				Hoarseness	.000	Vocal training	28%
				Tired voice	.004	Speech therapy treatment	5%
Vera García et al. (2022)	93 teachers from Spain (71% women)	Infant, Primary and Secondary	Acoustic recordings in reading	Public school	<.001
				Second trimester	<.001
				Third trimester	<.001
Żurek et al. (2022)	128 teachers from Poland (87.5% women)	Infant, Primary, Secondary and Superior	Questionnaire based on previous studies	Number of classes	.012	Absenteeism	30%
				Stationary work	.043

On the other hand, 11 studies employed questionnaires based on previous studies, such as the Voice Handicap Index (VHI) by Jacobson et al. (1997) and the Vocal Fatigue Index (VFI) by Nanjundeswaran et al. (2015). However, several studies did not specify the exact version or adaptation of the instruments used. In addition, 8 studies used self-constructed questionnaires as data collection tools. Acoustic recordings and environmental measurements were also used in 3 studies through programs such as Praat (Boersma, 2004). For more details, see Table 4.

It is important to note that the studies included in this review presented some degree of qualitative heterogeneity, particularly with regard to study design, operational definitions of key variables (e.g., “vocal fatigue” or “voice disorder”), and sample characteristics such as teaching level, gender distribution, and cultural context. Most of the included studies followed cross-sectional observational designs based on self-report questionnaires (n = 18), while a smaller number incorporated observational designs with repeated objective measurements, mainly acoustic recordings conducted at different moments of the workday or academic term (n = 3). No experimental, quasi-experimental, or interventional studies were included, in accordance with the predefined exclusion criteria. These sources of heterogeneity should be considered when interpreting the prevalence and strength of the identified predictors and consequences.

Predictors

Numerous predictors were found significantly related to the appearance of voice disorders. These associations are primarily grounded in the cumulative occupational vocal demands experienced by teachers, which increase the risk of vocal fatigue, laryngeal muscle tension, and mucosal irritation due to prolonged voice use under unfavorable acoustic or organizational conditions.

Predictors were grouped into three main semantic categories: voice symptoms, personal risk factors, and occupational risk factors (environmental and organizational). Within each category, the predictors are presented in descending order of frequency according to their occurrence in the reviewed literature.

Voice symptoms were reported as predictors in 11 of the 21 included studies. The most frequently described symptoms were vocal fatigue, hoarseness, dry throat, vocal effort, pain and discomfort in the throat, as well as burning sensation and need to clear throat. However, symptoms such as itching in the throat, a feeling of choking, vocal tension, a broken voice, cough and difficulty in projection and vocal control, as well as the sensation of oppression and knot in the throat, were also described (Evitts, Allebeck, & Åberg, 2022; Jonsdottir, Rantala, Oskarsson, & Sala, 2015; Leão, Oates, Purdy, Scott, & Morton, 2015; Sapir, Keidar, & Mathers-schmidt, 1993).

Personal risk factors were identified in 16 studies, including female gender, stress, respiratory infections, and common mental disorders such as anxiety and depression (Alrahim, Alanazi, & Al-Bar, 2018; Alva, Machado, Bhojwani, & Sreedharan, 2017; da Rocha & de Mattos Souza, 2013; Jonsdottir et al., 2015; Sapir et al., 1993). Relevant personal factors such as the presence of allergies, asthma, hearing impairment, Gastroesophageal Reflux (GER), sleep problems and family history, as well as educational level, age and alcohol consumption were also reported (Ählander, Rydell, & Löfqvist, 2011; Alshuhayb et al., 2022; Mesquita de Medeiros et al., 2012). There appears to be no consensus regarding tobacco consumption, since three studies defended that smoking was a significant predictor (Alrahim et al., 2018; Kyriakou, Theodorou, Petinou, & Phinikettos, 2020; RoscellaInja, 2016) while two studies stated the opposite (Alshuhayb et al., 2022; Lee, Kim, & Lee, 2018). Similarly, inconsistent findings were observed for gender, with most studies identifying female teachers as at higher risk, although a few did not confirm this association.

Occupational risk factors were the most frequently reported predictors, appearing in 18 studies. These included both environmental factors and organizational factors. The most representative environmental occupational risk factor was environmental noise, which included noise from airplanes and/or the street, outside noise from children playing, noise inside the building, noise inside class and electronic noise (Cantor Cutiva & Burdorf, 2014; Kyriakou et al., 2020). Environmental factors such as strong odors, dust, air quality or echo were also reported (Ählander et al., 2011; Evitts et al., 2022).

The organizational work factors found were the number of students, the number of classes, the number of work hours, the number of years worked and working in public school, as well as in nursery, primary and secondary school (Kyriakou et al., 2020; Rute Sanabria, Atará-Piraquive, & Cantor-Cutiva, 2023; Van Houtte et al., 2011). It was also related to being a classroom teacher, teaching physical education, teaching classes above 4th grade, conducting morning sessions, using vocal discipline, having short breaks, and working in person (da Rocha et al., 2017; Lee et al., 2018; Żurek, Jasak, & Rzepakowska, 2021).

Acoustic indicators

In contrast to subjective symptoms and self-reported predictors, several studies incorporated acoustic measures as objective indicators of vocal function, reflecting physiological changes associated with sustained or maladaptive vocal demands rather than causal risk factors. These indicators were used to characterize alterations in voice production once vocal difficulties were already present.

The acoustic indicators most associated with voice disorders were fundamental frequency (F0), jitter, and shimmer, followed by alpha ratio and sound pressure level (SPL) (Laukkanen, Ilomäki, Leppänen, & Vilkman, 2008; Rajasudhakar & Savithri, 2010). These objective measures reflect perturbations in frequency and intensity after the workday due to sustained vocal demands, supporting the perceptual evidence of vocal fatigue in teachers.

Consequences

There is a high number of negative and frequent consequences associated with voice disorders. The best known consequences are absenteeism and sick leave. On the one hand, absenteeism from work was studied in eight articles, whose prevalence ranged from 3.32% to 45% (Alrahim et al., 2018; Alshuhayb et al., 2022; Cantor Cutiva & Burdorf, 2014; Kyriakou et al., 2020; Mesquita de Medeiros et al., 2012; Sapir et al., 1993; Van Houtte et al., 2011; Żurek et al., 2021). On the other hand, sick leave was analyzed in four articles, whose prevalence varied between 15% and 35% (Ählander et al., 2011; da Rocha & de Mattos Souza, 2013; da Rocha et al., 2017; Jonsdottir et al., 2015). In turn, the period in which professionals are absent from work can be classified as short or long leaves of absence, depending on the duration of the sick leave reported by each study. Short leaves generally referred to absences of less than 15 days, while long leaves exceeded this duration. Three studies showed that short leaves of absence had a prevalence of 33–75%, while long leaves of absence had a prevalence of 16–30% (Leão et al., 2015; Mesquita de Medeiros et al., 2012; Van Houtte et al., 2011).

In contrast to absenteeism, presenteeism is defined as that period of attendance in which the teacher attends even knowing that they should be absent. Workplace presenteeism was studied in three articles, whose prevalence was around 50–87% (Leão et al., 2015; Lee et al., 2018; RoscellaInja, 2016). Furthermore, eight studies determined that between 5% and 50% of teachers sought professional help for their vocal problems, of which between 1% and 3% requested speech therapy treatment (Ählander et al., 2011; Alshuhayb et al., 2022; Cantor Cutiva & Burdorf, 2014; Kyriakou et al., 2020; Leão et al., 2015; RoscellaInja, 2016; Sapir et al., 1993; Van Houtte et al., 2011). Likewise, a study by da Rocha and de Mattos Souza (2013) showed that 68% preferred to perform vocal rest instead of seeking medical help. Similarly, three articles reported that between 19% and 30% of teachers had completed voice-related training (Alshuhayb et al., 2022; RoscellaInja, 2016; Van Houtte et al., 2011).

On the other hand, two studies estimated that voice problems in teachers represented a loss of work productivity of between 41% and 69%, mainly due to reduced teaching efficiency, classroom voice limitations, and increased fatigue (Cantor Cutiva & Burdorf, 2014; Kyriakou et al., 2020). Furthermore, a study showed that 36% of teachers with voice disorders suffered a negative professional impact, particularly in terms of reduced participation in group activities, avoidance of speaking tasks, and decreased career satisfaction (Sapir et al., 1993). In turn, the study by Alva et al. (2017) found that 26% of teachers with voice problems experienced social isolation. Finally, these authors reported that only 15% of teachers with voice disorders reported a satisfactory quality of life.

Methodological quality of the included studies

Of the studies assessed for methodological quality, 26 were rated as excellent and one as good according to the adapted CRF-QS criteria. Minor limitations were identified in some studies, mainly related to an insufficient review of the existing literature. Table 5 presents the quality assessment scores for each study.

Table 5.

Results of the methodological quality evaluation based on the CRF-QS (Law et al., 1998).

Article	1)*	2)*	3)*	4)*	5)*	6)*	7)*
Authors (year)	Purpose of the study	Literature	Sample	Results	Conclusions	Score	Interpretation
Ählander et al. (2011)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Alrahim et al. (2018)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Alshuhayb et al. (2022)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Alva et al. (2017)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Cantor Cutiva and Burdorf (2014)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Cantor Cutiva and Burdorf (2015)	Yes	Yes	No	Yes	Yes	80%	Excellent
da Rocha et al. (2017)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
da Rocha and de Mattos Souza (2013)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Evitts et al. (2022)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Jonsdottir et al. (2015)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Kooijman et al. (2005)	Yes	Yes	No	Yes	Yes	80%	Excellent
Kooijman et al. (2006)	Yes	Yes	No	Yes	Yes	80%	Excellent
Kyriakou et al. (2020)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Laukkanen et al. (2008)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Leão et al. (2015)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Lee et al. (2018)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Mesquita de Medeiros et al. (2012)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Nerrière et al. (2009)	Yes	Yes	No	Yes	Yes	80%	Excellent
Rajasudhakar and Savithri (2010)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
RoscellaInja (2016)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Rute Sanabria et al. (2023)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Sapir et al. (1993)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Schneider et al. (2006)	Yes	No	No	Yes	Yes	60%	Good
Smolander and Huttunen (2006)	Yes	Yes	No	Yes	Yes	80%	Excellent
Van Houtte et al. (2011)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Vera García et al. (2022)	Yes	Yes	Yes	Yes	Yes	100%	Excellent
Żurek et al. (2022)	Yes	Yes	Yes	Yes	Yes	100%	Excellent

Notes: *1) Purpose of the study clear; *2) Review of relevant and sufficient literature; *3) Detailed description of the sample; *4) Use of statistical significance terms; *5) Conclusions consistent with the method and results; *6) Score based on the previous parameters; *7) Interpretation of scores based on Faber et al. (2015).

Discussion

The objective of this study was to carry out a comprehensive review of the main predictors associated with voice disorders and the consequences that these disorders generate in the teaching population. The results showed that the most common predictors expressed by teachers are voice symptoms such as vocal fatigue, hoarseness or vocal effort, occupational risk factors such as environmental noise or the number of students, and emotional aspects such as stress or anxiety. Likewise, a high prevalence of absenteeism and sick leave was observed among teachers in relation to voice problems. These findings are consistent with previous systematic reviews reporting that teachers are among the professional groups with the highest prevalence of voice disorders worldwide (Byeon, 2019; Hunter & Banks, 2017). Similarly, several studies have highlighted the significant role of psychosocial stress, excessive vocal demands, and poor acoustic environments as key contributors to the onset of dysphonia in teachers (Cantor Cutiva & Burdorf, 2015; Vertanen-Greis, Löyttyniemi, & Uitti, 2020).

Regarding the predictors, two issues should be clarified. First, a large number of articles determined that being female was considered a personal risk factor. However, taking into account that most or all of the participants were women, it would be necessary to determine whether this constitutes a genuine predictor or is merely a statistical artifact. This pattern has also been discussed by Hunter and Banks (2017) and Roy et al. (2004), who suggest that biological and hormonal differences may increase susceptibility, but methodological imbalance between male and female samples often amplifies this association.

Secondly, inconsistencies have been found when determining whether smoking—or, conversely, not smoking—is considered a predictor. While some studies reported no significant association, this is difficult to reconcile with established evidence: smoking is a major etiological factor for laryngeal mucosal disease, including Reinke's edema, and it measurably worsens acoustic parameters such as fundamental frequency stability and perturbation measures (Garcia Martins, Mendes Tavares, & Benito Pessin, 2017; Pinto, Crespo, & Mourão, 2014). Therefore, it is more plausible that methodological limitations—such as small sample sizes, self-reported smoking habits, or unmeasured confounders—explain these contradictory findings rather than any protective effect of smoking. Considering the numerous harmful effects of tobacco on general and vocal health, such as mucosal irritation, respiratory disease, and cancer (Lugones Botell, Ramírez Bermúdez, Pichs García, & Miyar Pieiga, 2006), it is coherent to interpret smoking as a risk factor for voice disorders rather than the opposite.

Regarding the consequences, a high prevalence of work absenteeism has been observed as well as work presenteeism. The fact that teachers do not leave their jobs despite being unwell may be due to cultural reasons such as work commitment, economic reasons such as the absence of substitute teachers or personal reasons such as a sense of conscience (Lee et al., 2018; Rute Sanabria et al., 2023). Furthermore, the economic impact of absenteeism and presenteeism has also been documented in occupational health research, where reduced productivity and recovery periods contribute to a significant social cost (Cantor Cutiva & Burdorf, 2015).

Finally, a low prevalence has been observed when seeking speech therapy treatment, which may be due to teachers’ misinformation about the profession or their lack of awareness of the importance of voice disorders. Similar barriers to treatment-seeking have been reported by Alves Silverio et al. (2008) and Meier and Beushausen (2021), who emphasize the need for preventive programs integrated into teacher training curricula. On the contrary, numerous studies have shown the effectiveness of vocal training programs based on vocal hygiene guidelines, prevention measures, and semi-occluded vocal tract exercises (Duffy & Hazlett, 2004; Guzman et al., 2013).

An important aspect when interpreting these findings is the methodological heterogeneity observed across the included studies, particularly in relation to study design and measurement approaches. Most studies followed cross-sectional observational designs based on self-report questionnaires to identify vocal symptoms, predictors, and consequences, whereas a smaller number incorporated objective acoustic indicators obtained through repeated measurements across the workday or academic period. These differences reflect distinct but complementary approaches to assessing vocal health.

This heterogeneity has direct implications for the generalization and comparison of epidemiological estimates. Variability in operational definitions of key constructs (e.g., vocal fatigue, voice disorder), differences in instrument sensitivity, and the absence of standardized cut-off values limit comparability across countries, educational contexts, and teaching levels. Moreover, while acoustic indicators provide valuable objective information about vocal function, they represent indicators of vocal fatigue or maladaptive vocal demand responses rather than causal risk factors, which complicates synthesis beyond descriptive aggregation. As a result, differences in reported prevalence and effect sizes may partially reflect methodological variability rather than true population-level differences.

The limitations of this study lie mainly in two issues. Firstly, most of the studies were based on teachers’ perception questionnaires and not on objective and quantifiable measures, so their reliability may not be sufficiently robust. Secondly, there are certain studies that analyzed very small samples, which makes it impossible to generalize their results. In addition, qualitative heterogeneity in study designs and definitions of key variables (e.g., vocal fatigue, dysphonia) must be acknowledged as an inherent limitation (Hunter & Banks, 2017).

Despite these limitations, this study provides access to abundant and updated information on the predictors and consequences of voice disorders, which will be useful for conducting future research related to this topic. Although acoustic measures provide valuable objective data and can serve as early indicators of vocal fatigue or maladaptive vocal demand responses, their preventive use remains limited. The lack of standardized protocols and longitudinal follow-up studies makes it difficult to determine precise cut-off values that predict future voice disorders. Therefore, acoustic assessment should be integrated with perceptual and self-reported tools within a comprehensive vocal risk monitoring approach. In this way, the need for studies focused on the development of speech therapy programs based on the prevention of voice disorders is confirmed. Preventive interventions should not only address individual vocal behavior but also organizational conditions such as workload, rest periods, and classroom acoustics (Alharbi et al., 2024; Vertanen-Greis et al., 2020). Ultimately, improving vocal health among teachers represents both a clinical and occupational priority, with direct implications for educational quality and public health.

Conclusion

This systematic review identified a set of predictors and consequences of voice disorders in teachers that are consistent across the literature. The most recurrent and strongly supported predictors include psychological stress, respiratory infections, common mental disorders, environmental noise, and organizational factors such as the number of students, class load, and years of teaching experience. Likewise, the most prevalent symptoms were vocal fatigue, hoarseness, throat dryness, vocal effort, discomfort, and pain, which together reflect the cumulative impact of sustained vocal demands and occupational conditions.

Regarding the consequences, absenteeism, presenteeism, and loss of productivity emerged as the most frequent outcomes, often accompanied by a decline in quality of life, social isolation, and reduced professional performance. These findings highlight that voice disorders in teachers constitute not only a clinical but also an occupational and social concern.

From a practical perspective, the results emphasize the need to integrate vocal health promotion and prevention programs into teacher training and workplace health policies. Future research should aim to standardize acoustic and perceptual assessment protocols, develop longitudinal studies that monitor vocal risk over time, and evaluate the effectiveness of preventive interventions in real educational settings. These findings reinforce the importance of developing comprehensive prevention frameworks that integrate clinical, educational, and occupational perspectives to promote sustainable vocal health in teachers.

Authors’ contributions

Zaira Ortega: conceptualization, data collection, analysis, and manuscript drafting.

Jennifer Balade: supervision, methodological guidance, and critical manuscript revision.

Both authors approved the final version of the manuscript.

Ethical considerations

This study is a systematic review of previously published research and did not involve human participants or animal subjects; therefore, ethical approval was not required.

Informed consent

Not applicable, as no human participants were involved.

Use of artificial intelligence

Artificial intelligence tools were used exclusively for language editing and formatting assistance. All content was critically reviewed and approved by the authors, who take full responsibility for the final manuscript.

Funding

This work was supported by a predoctoral fellowship (FPU) from the Ministry of Science, Innovation and Universities, Spain.

Conflict of interest

The authors declare no conflict of interest.