Heat stress has emerged as an important occupational health challenge in the context of rising ambient temperatures, prolonged heat waves, and increasing frequency of extreme weather events.[1] It is a problem that happens when a body cannot get rid of the heat that is generated by metabolism and the environment in which it lives, causing physiological heat strain and potentially heat exhaustion, heat stroke, renal dysfunction, and death in extreme cases.[2] Outdoor workers are especially vulnerable due to the combination of direct sunlight exposure, high ambient temperature and humidity, physical exertion, and extended working hours.[3] Worldwide, over 2.4 billion workers are estimated to be exposed to excessive heat, and over 22.85 million occupational injuries are estimated to be linked to heat exposure each year.[4] Furthermore, physiological heat strain is greater than 1/3 of individuals who routinely work in hot environments.[5]

This is especially important for occupations like agriculture, construction, road maintenance, transportation, and municipal services, as well as other manual outdoor work.[6] The meta-analysis of 38 field studies with 2,409 outdoor workers in 41 occupations and 21 countries indicated that occupational heat stress significantly affects core and skin temperature, heart rate, and urine specific gravity, and significantly decreases workers' ability to do physical work.[7] It is important to note that occupational exposures to excessive heat are not exclusively associated with extreme heatwaves, as about 90% of occupational exposures take place outside of the heat wave window, indicating routine occupational heat exposure is likely a much greater occupational health problem than episodic heat emergencies.[1]  Heat exposure also causes sweating and loss of body fluids, and dehydration can be a significant physiological response in workers who are exposed to chronic heat stress.[8] Research conducted among outdoor workers has found significant sweat losses and shown a relationship between rising environmental heat, dehydration, and heat-related symptoms.[9, 10]

The systemic effects of occupational heat stress have been studied, but there are fewer studies focused on the effects of occupational heat stress on oral health and salivary function. Saliva plays a vital role in the maintenance of oral homeostasis by lubricating the oral cavity, buffering, providing antimicrobial protection, remineralization, taste, mastication, swallowing, and speech. Xerostomia is a subjective sensation of oral dryness, and hyposalivation is objectively decreased salivary flow, with these two conditions not being synonymous. Reduced salivary protection may be associated with eating and swallowing problems, changes in taste, bad breath, mouth discomfort, tooth decay, gum disease, and other oral issues, such as candidiasis. Xerostomia does not only affect older adults:[11] A recent report estimated that 8.8% of 2,077 young adults (18-24 years) reported xerostomia, and a recent meta-analysis estimated the prevalence of xerostomia to be around 21% in older adults.[12, 13]

There is a biologically acceptable relationship between xerostomia and occupational heat exposure. Excessive exposure to hot temperatures may lead to higher sweat rates and insensible water loss, which can result in dehydration and changes in salivation. Dehydration might lead to a decrease in unstimulated salivation and a greater awareness of oral dryness with prolonged dehydration. A 2026 study correlated unstimulated salivary flow with temperature, humidity, and hydration status of the environment, supporting this relationship.[12]  However, that study was conducted among healthy young men under indoor conditions and found that subjective xerostomia was not directly explained by temperature or hydration alone, suggesting that the relationship between environmental heat, dehydration, salivary function, and perceived oral dryness may be complex.[12] Thus, further direct evidence obtained in occupational heat exposure is still required.

Outdoor workers are an important population in which this relationship can be studied as a result of simultaneous exposure to heat, physical exertion, sweating, and lack of fluid replacement during regular work. However, although occupational heat stress has been recognized as a systemic health threat, the potential for it to be expressed as a common but under-reported oral symptom (xerostomia) has received relatively little attention. The identification of xerostomia as a potentially simple and early indicator of inadequate hydration or heat strain, rather than just a systemic outcome of exposure to the heat, would extend the understanding of morbidity due to heat exposure beyond systemic effects. This evidence can also be used to inform workplace hydration planning and interventions for oral health. This study therefore set out to identify whether there was a relationship between occupational heat stress and xerostomia among outdoor workers and whether increasing levels of heat stress exposure correlated with subjective oral dryness.

This was an analytical cross-sectional study carried out at Bahria University Health Sciences, Karachi, from 1st February 2026 to 31st July 2026. The sample size was determined with the OpenEpi software program for estimating a single population proportion. The proportion of dry mouth was assumed to be 45.9% as reported in a previous study of construction workers exposed to heat. With these parameters, the minimum sample size calculated was 149 participants, with an expected prevalence of 45.9% and 8% absolute precision, assuming a 95% confidence level.[14] The sample size was adjusted to 164 participants after adding about 10% to account for possible non-response or incomplete data. A non-probability consecutive sampling technique was used. These included workers ≥18 years of age with 6 months or more of outdoor occupational exposure and who had some exposure to ambient environmental heat during their workday. All participants were workers with informed consent who worked outdoors for a minimum of four hours per working day. Patients who had previously been diagnosed with a disorder of the salivary glands (Sjögren syndrome), head and neck malignancy, or radiotherapy to the head and neck were excluded. Other patients receiving drugs which are known to significantly decrease salivation such as anticholinergic drugs or other drugs with significant xerogenic properties, were also excluded. Excluded were workers who had uncontrolled systemic disease that might separately impair salivary function, workers with active oral disease and/or extensive oral mucosal disease, and workers who were unable to fill out the questionnaire. Fasting or drinking alcohol within 2 hours prior to assessment was also excluded, as it may affect hydration level and salivary secretion. An instrument consisting of a structured questionnaire, which was prepared after reviewing the literature pertaining to occupational heat stress, hydration, and xerostomia, was used to collect data. The questionnaire was first pilot-tested with a few of the workers who were not included in the study sample, and modifications were made to clarify and improve its comprehension. The following information was collected: age, sex, and educational level, occupation, length of time working, average daily hours spent outdoors, and use of protective clothing, access to drinking water, frequency of water consumption, smoking, and medical and medication history relevant to the work. Occupational heat exposure was assessed during the participants' working hours. Ambient temperature and relative humidity were measured at the job site, and wet-bulb globe temperature (WBGT) was used as the primary environmental measure of heat exposure at the job site when it could be used. WBGT is extensively used in occupational heat-stress research, such as studies of outdoor workers.[10, 15] Xerostomia, thirst, weakness, headache, muscle cramps, dizziness, and fatigue were also evaluated among the participants as heat-related symptoms. The hydration status was measured by a standardized hydration indicator when available, such as urine specific gravity, since urine specific gravity has been broadly used in occupational heat stress studies as an objective measure of hydration.[16] A validated subjective xerostomia questionnaire was used to evaluate xerostomia. The participants were asked about the symptoms of persistent oral dryness, sipping of liquids for oral manipulation, inability to eat dry foods, dry lips or throat, and reduced saliva sensation. The xerostomia severity was scored using the selected xerostomia questionnaire. Unstimulated whole saliva was also gathered whenever possible to objectively evaluate salivary flow in order to differentiate subjective xerostomia from actual salivary hypofunction. This was significant because xerostomia is the subjective experience of oral dryness, while hyposalivation is an objectively diminished salivary flow. The data collected were analyzed, cleaned, and entered in SPSS version 26. Continuous variables were normally distributed by the Shapiro-Wilk test and were presented as mean ± standard deviation or median (interquartile range) for those that were not normally distributed. Categorical variables were displayed as frequencies and percentages. Prevalence of xerostomia was computed, and xerostomia distribution was examined according to occupational heat exposure levels. The independent-samples t-test and Chi-square test were used for comparison between workers with and without xerostomia, depending on the nature of the variable. The association between heat stress and xerostomia was initially evaluated using univariate analysis. Variables showing a clinically relevant association or a p-value ≤0.20 on univariate analysis were considered for multivariable analysis. Binary logistic regression was subsequently performed to determine independent factors associated with xerostomia after adjustment for potential confounders such as age, sex, smoking, duration of outdoor work, working hours, fluid intake, and hydration status. Odds ratios were reported with 95% CIs, adjusted. A p-value of ≤0.05 was set to be considered statistically significant.

A total of 164 outdoor workers were included, with a mean age of 37.8±9.6 years. Most of them were male (92.1%), and most were involved in the security guard (29.3%) and gardener (18.9%) occupations. Mean years of employment were 7.2±5.1, and mean working time outdoors was 6.8±1.5 h/day. Regular access to drinking water was reported by 80.5% of workers, while 32.9% were smokers. (Table 1)

The mean WBGT was 31.4±2.7°C, indicating that 27.4% of the workers had high heat exposure. 41.5% and 20.1% of the participants had mild or moderate/severe dehydration. Excessive sweating (62.8%), excessive thirst (58.5%), weakness/fatigue (51.2%), and headache (43.3%) were the most frequently reported heat-related symptoms. (Table 2)

Xerostomia was reported by 73 (44.5%) workers, including 39 (23.8%) with mild, 25 (15.2%) with moderate, and 9 (5.5%) with severe symptoms. Dry lips or throat was the most common single symptom (46.3%). Salivary unstimulated flow rate (Uf) was 0.39±0.18 mL/min, with 31.1% of the participants having hyposalivation. (Table 3)

Xerostomia increased significantly with increasing heat exposure, from 25.0% among workers with lower exposure to 40.3% with moderate exposure and 73.3% with high exposure (p<0.001). Xerostomia was also significantly more common among workers who work >6 hrs/day, who experience greater dehydration, who drink <3 L/day, and smokers. Protective clothing, employment duration >5 years, and age ≥40 years were not significantly associated with xerostomia. (Table 4)

The mean outdoor working hours, WBGT, and daily fluid intake were significantly higher, and mean unstimulated salivary flow was significantly lower among workers with xerostomia compared to workers without xerostomia. They also had a higher urine specific gravity, which was a measure of their dehydration. Workers experiencing xerostomia had a modestly greater duration of employment, while age was not significantly different between groups. (Table 5)

Multivariable analysis showed that high heat exposure was independently associated with xerostomia (p=0.001). Working outdoors for >6 hours/day (p=0.024), moderate/severe dehydration (p=0.003), and fluid intake <3 L/day (p=0.039) also remained independently associated with xerostomia. Moderate heat exposure, smoking, and length of service were not statistically significant independent predictors, as was age. (Table 6)

Table 1. Sociodemographic and occupational characteristics of outdoor workers (n=164)

Table 2. Heat exposure, hydration status and heat-related symptoms among outdoor workers (n=164)

Table 3. Xerostomia and salivary findings among outdoor workers (n=164)

Table 4. Association of occupational heat exposure and other factors with xerostomia

Table 5. Comparison of continuous heat and salivary parameters according to xerostomia status

Table 6. Multivariable logistic regression analysis of factors associated with xerostomia

The present study demonstrated a substantial burden of xerostomia among outdoor workers, with 44.5% reporting xerostomia. In addition, occupational heat exposure had a clear exposure-response relationship with xerostomia: the prevalence of xerostomia rose from 25.0% for low heat exposure, to 40.3% for moderate heat exposure, and 73.3% for high heat exposure (p<0.001). High heat exposure remained independently associated with xerostomia after adjustment for potential confounders, as workers in high heat had nearly five times greater odds of xerostomia than those in low heat (p=0.001). These findings suggest that occupational heat exposure may contribute substantially to oral dryness among outdoor workers and that the association cannot be explained solely by age, smoking, duration of employment, working hours, or fluid intake. The prevalence of xerostomia observed is biologically plausible given the great amount of thermal stress experienced by the study population. The mean WBGT was 31.4±2.7°C, while 27.4% of workers were classified as having high heat exposure. A 2021 study of 934 stone-quarry workers in India reported particularly pronounced heat-related symptoms during summer, including excessive sweating in 93.5% and thirst/dry mouth in 88.7% of workers. The previous study did not diagnose xerostomia as an occupational symptom, but the results align with the association between occupational heat exposure and oral dryness seen in the present study.[17] In a similar study conducted in the construction industry in 2021, working in hot conditions outdoors was found to be linked to a higher risk of developing a heat-related illness, indicating that environmental and personal factors both play a role in occupational heat strain.[18] The results in this study are also consistent with the overall occupational heat literature. In a comprehensive review and meta-analysis of 38 field studies that included 2,409 outdoor workers from 21 countries, Ioannou et al. (2022) reported that occupational heat stress influenced occupational indicators of heat strain and urinary specific gravity, which are markers of hydration status.[7] In the present study, 61.6% of workers were either mildly or moderately/severely dehydrated, and mean urine specific gravity was significantly higher among workers with xerostomia than among those without xerostomia (p<0.001). Therefore, the objective hydration results obtained in our study offer a possible link between environmental heat exposure and oral dryness on a physiological basis. The relationship between heat exposure and dehydration was further supported by studies conducted among agricultural workers. A 2023 study of farmworkers in Thailand demonstrated substantial seasonal variation in WBGT and hydration-related parameters, with the highest environmental heat exposure occurring during summer when mean WBGT reached 38.1°C.[19] More recently, a 2025 study of 200 male agricultural workers in eastern India found that WBGT predicted physiological heat strain and that dehydration increased from 16.5% to 33% during the work period; excessive sweating was also associated with increased dehydration risk.[9] These results are similar to our results of 62.8% of workers who were experiencing excessive sweating and dehydration which showed a strong association with xerostomia. When we took moderate/severe dehydration as the individual variable in our regression model, the odds of xerostomia were increased by about 3.7-fold (p=0.003). The value of fluid intake was also illustrated, as those consuming less than 3 L of fluid per day had significantly higher odds of xerostomia. The prevalence of xerostomia was higher in workers who consumed <3 L/day (52.6%) than among those who consumed ≥3 L/day (25.0%) (p=0.006), and low intake of fluid was independently associated with xerostomia after adjustment (p=0.039). The result aligns with Hess et al. (2022), which showed that ad libitum drinking during physical activities in the heat does not lead to dehydration if the guidelines for occupational heat stress are followed.[20] Likewise, a 2024 study of migrant farmworkers highlighted the lack of access to water, shade, and rest breaks as key factors in dehydration during occupational heat exposure.[21] The length of heat exposure also seemed relevant in the current study. There was a significant difference between the prevalence of xerostomia among workers with >6 hours of outdoor work and those with ≤6 hours of outdoor work (p=0.003); the association between prolonged outdoor work and xerostomia was independent of other factors (p=0.024). This finding is in line with what has been found by studies that show that occupational heat exposure for a longer duration leads to greater physiological strain. In a study of 569 outdoor workers in El Salvador and Nicaragua in 2023, significant WBGT exposures were identified in multiple outdoor industries, and workers were often subjected to high thermal loads during the afternoon work hours and short break periods.[22] Thus, the present results indicate that the duration of exposure is also important for oral effects of heat stress as well as for systemic heat strain. This is especially supported by the latest 2026 observational study by Ito et al. in which they studied the temperature, humidity, hydration status, salivary flow, and xerostomia longitudinally within healthy young men. The authors concluded that unstimulated salivary flow rate was significantly related to temperature, humidity, and urine specific gravity, but xerostomia was not directly related to temperature or humidity.[12] Instead of just measuring the environmental temperature, we measured what we called “occupational heat exposure” through WBGT and showed a strong correlation between occupational heat exposure and clinically relevant xerostomia in an occupational population. The reduced salivary flow rates among the symptomatic workers are additional evidence that supports this hypothesis that dehydration-induced decrease in salivary flow may contribute to the subjective feeling of dryness. The results are significant, especially in the context of occupational health in a hot climate like Karachi. The study shows that the effects of heat exposure can be far-reaching, not just in terms of traditional heat-related symptoms and cardiovascular or renal strain, but also on oral health. High WBGT, long duration outside, poor fluid consumption, and dehydration can all affect salivary production and cause xerostomia. This is clinically important in that comfort, swallowing, eating, speaking, oral mucosal protection, and susceptibility to dental caries and other oral complications may be compromised as a result of xerostomia. Thus, the strategies for managing workplace heat should not only be geared toward preventing severe heat illness but should also take into account oral health and hydration issues. LIMITATIONS There were some limitations to the study. Its cross-sectional design prevented establishing a temporal or causal relationship between occupational heat exposure and xerostomia. The results may not be generalizable to other groups of people working in the same institution or to other occupational groups. The study sample was mostly male, so it could not be applied to female outdoor workers. The assessment of heat exposure, hydration, and salivary parameters were taken at one time and may not represent a cumulative or seasonal exposure. Dietary factors, oral hygiene, caffeine consumption and other behavioral factors were not ruled out as important confounders. Finally, the modest sample size might have reduced the confidence in detecting any weaker associations.

Occupational heat stress was strongly associated with xerostomia among outdoor workers. High heat exposure, prolonged outdoor working hours, dehydration, and lower fluid intake were independently associated with increased odds of xerostomia. The unstimulated salivary flow was also significantly lower for workers with xerostomia. These findings highlight oral dryness as an important potential manifestation of occupational heat exposure and support workplace interventions emphasizing adequate hydration, drinking-water availability, work-rest schedules, and heat-stress prevention.