This was a cross-sectional study of 17,726 male and female current and recent workers aged 18+ who participated in the 2023 NHIS adult sample (the entire adult sample included 29,522 participants). NHIS is a cross-sectional, nationally representative sample of the non-institutionalized U.S. civilian population.¹⁶ The 2023 survey included sponsored hearing-related questions and was a year when industry and occupation information were captured. Survey participants were identified as current or recent workers if they 1) answered ‘Yes’ to the question “Last week, did you work for pay at a job or business?”; or 2) answered ‘Yes’ to the question “Did you have a job or business last week, but were temporarily absent due to illness, vacation, family or maternity leave, or some other reason?”; or 3) answered “Within the past 12 months” to the question “When was the last time you worked for pay at a job or business, even if only for a few days?”. Workers who answered that they were ‘deaf’ to the hearing difficulty question were excluded (89 workers) as this level of hearing impairment is unlikely to have been caused predominately by occupational exposures.¹⁷

Weighted prevalences and weighted adjusted prevalence ratios (PRs) of self-reported hearing difficulty and self-reported workplace exposures to ototoxic chemicals, noise and a combination of these exposures were estimated by industry, occupation and demographic. The PRs are weighted adjusted risk estimates compared with a reference group. Workplace exposure to secondhand smoke was examined combined with other ototoxic chemicals, but also separately. The hearing difficulty and occupational exposure questions are described in the next section. NHIS participants reported industry and occupation verbatim and these responses were coded by U.S. Census Bureau coding specialists into 2017 North American Industry Classification System (NAICS) codes and 2018 Standard Occupation Classification (SOC) Codes.^18,19 The covariates of interest were age group, sex, education, race/ethnicity, obesity determined from body mass index (BMI) and smoking status (current, former and never smoker). Races other than ‘Hispanic’ are all non-Hispanic.

This study adhered to “Strengthening the Reporting of Observational Studies in Epidemiology (STROBE)” guidelines (Supplemental Digital Content, http://links.lww.com/JOM/B982). The 2023 NHIS was approved by the Research Ethics Review Board of the National Center for Health Statistics and the U.S. Office of Management and Budget. All 2023 NHIS participants consented orally prior to participating. This study was reviewed by the Centers for Disease Control and Prevention (CDC), deemed not research, and was conducted consistent with applicable federal law and CDC policy. §§ See e.g., 45 C.F.R. part 46.102(l)(2), 21 C.F.R. part 56; 42 U.S.C. §241(d); 5 U.S.C. §552a; 44 U.S.C. §3501 et seq.

All the data used in this study were self-reported, including the outcome and exposures. The outcome was hearing difficulty, determined from the question below. Workers who identified their hearing ability as “a little trouble hearing”, “moderate trouble” or “a lot of trouble” were identified as having hearing difficulty.

These next questions are about your hearing WITHOUT the use of hearing aids or other listening devices.

Without a hearing aid or other listening devices, is your hearing excellent, good, a little trouble hearing, moderate trouble, a lot of trouble, or are you deaf?

Below are the questions for occupational exposures to ototoxic chemicals and secondhand smoke. In this study, workers identified to have ototoxic chemical exposure answered yes to questions A & B, or yes to C, or yes to A, B and C. All workers with secondhand smoke exposure were included in the estimates for ototoxic chemicals. Workers exposed to secondhand smoke were also examined separately. The “Read if necessary” part of the questions were read to survey respondents if they had trouble understanding the question.

The next set of questions ask about your exposure to chemicals and tobacco smoke in your job.

A. During the past 12 months, did you work in a job where you were exposed to any of the following: solvents, industrial glues, heavy metals, pesticides or motor engine exhaust?

* Read if necessary: Solvents are liquids, not including water, used for dissolving other substances, such as dry-cleaning fluids, thinners, components of paints, industrial cleaners and paint removers. Industrial glues are adhesives that include acrylic, epoxy, hot melt, polyurethane, silicone, thermoset and industrial sealants. Heavy metals include lead, nickel, mercury, cadmium, arsenic, chromium, zinc and copper.

B. [If YES to A] Were you exposed for 4 or more hours a week?

C. During the past 12 months, did you work in a job where you were exposed for 4 or more hours a week to tobacco smoke from other people?

* Read if necessary: Tobacco smoke includes smoke from cigarettes, cigars, pipes, cigarillos, and hookahs. Do not include e-cigarettes or vaping devices that do not contain tobacco.

Analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC) and SUDAAN version 11 (RTI International, Research Triangle Park, NC) statistical software. Weighted prevalence estimates were obtained for the outcome (hearing difficulty) and each occupational exposure using the SAS surveyfreq procedure, incorporating NHIS sample weights. Estimates with a relative standard error ≥30% and ≤50% and their associated PRs were marked as such and estimates with a relative standard error ≥50% and their associated PRs were not reported. PRs (weighted adjusted risk estimates) with 95% confidence intervals were obtained using the SUDAAN procedure rlogist and adjusted for age group, sex, education, race/ethnicity, BMI and smoking status.

The PR reference groups were female for sex, 18–25 for age group, Asian for race/ethnicity, college degree for education, not obese for BMI, and never smoker for smoking status, as these groups have lower risks for hearing difficulty. Finance and Insurance was designated as the reference industry for the industry analyses, and Business and Financial Operations was assigned as the reference occupation for the occupation analyses, because these were used as the reference groups in previous analyses of NHIS data and have low prevalences of hearing difficulty and occupational exposures.^15,20

The attributable fraction [AF = (PR-1)/PR] was calculated for each exposure to identify the number of excess cases of hearing difficulty among the exposed [# excess cases = AF x total weighted prevalence of workers with exposure and hearing difficulty]. When a causal relationship has been established between an exposure and hearing difficulty, the number of excess cases among the exposed is also the number of cases that can be prevented if the exposure is reduced to a safe level. There is an established causal relationship between noise and hearing difficulty.² Although a causal relationship has been established between some ototoxic chemicals and hearing difficulty, some chemicals (e.g., secondhand smoke) may not cause hearing difficulty on their own, but rather exacerbate the effects of noise. Thus, only the number of excess cases among the exposed can be identified for ototoxic chemicals and secondhand smoke in this study.

Within the past year, 13% of workers were exposed to ototoxic chemicals, including 5% exposed to secondhand smoke (one substance containing ototoxic chemicals). The prevalence of ototoxic chemical exposure was three times higher among men and highest in the youngest age groups (18–25, 26–35). Asian workers had markedly lower exposures than the other races/ethnicities. Ototoxic chemical exposures were slightly lower among workers with some college but were otherwise fairly similar across levels of education. For secondhand smoke, the prevalence of exposure decreased as level of education increased. Obese workers had a slightly higher prevalence of exposure (15%) than those not obese (12%) and current smokers had the highest prevalence of exposure among all the levels of all demographics (26%).

Industry statistics are provided in Table 2. Industries with the highest prevalences of ototoxic chemical exposures were: Mining (49%); combined Mining and Oil and Gas Extraction (OGE) (44%); Construction (30%); Agriculture, Forestry, Fishing and Hunting (AFFH) (29%); Transportation (24%); and Manufacturing (24%). Finance and Insurance had the lowest prevalence of exposure (3%). Figure 1 depicts industries ranked by prevalence of ototoxic chemical exposure. Industries with the highest prevalences of secondhand smoke somewhat coincided: Mining (25%), combined Mining and OGE (22%); Warehousing (17%); Construction (14%); Arts, Entertainment, and Recreation (11%); and Utilities (9%). Secondhand smoke was lowest in Professional, Scientific and Technical Services (1%).

Occupation statistics are provided in Table 3. Occupations with the highest prevalences of ototoxic chemical exposures were: Installation, Maintenance, and Repair (43%); Farming, Fishing, and Forestry (39%); Construction and Extraction (35%); Production (30%); Protective Service (26%); and Transportation and Material Moving (23%). The same six occupations had the highest prevalences of secondhand smoke, in a different rank order. Ototoxic chemical exposures and secondhand smoke exposures were lowest in Computer and Mathematical (2%) and Educational Instruction and Library (1%), respectively.

Sixteen percent of workers were exposed to noise within the past year and 28% had a history of noise exposure (Table 1). The prevalence of noise exposure and a history of noise exposure were twice as high among men. Noise exposure was most prevalent in the youngest age groups (18–25, 26–35), while a history of noise exposure was fairly similar across age groups. Noise exposure and a history of noise exposure were much lower among Asian workers compared to workers of other races/ethnicities. Noise exposure was lower among workers with some college or a college degree, while a history of noise exposure was lower among those with a college degree. Obese workers had a higher prevalence of noise exposure and a history of noise exposure (18% and 33%) than those not obese (14% and 25%). Current and former smokers had the highest prevalences of noise exposure and a history of noise exposure among all levels of all demographics (45% and 39%).

Industries with the highest prevalences of noise exposure were: Mining (56%); combined Mining and OGE (52%); Manufacturing (38%); Construction (37%); Warehousing (33%); and AFFH (30%) (Table 2). Figure 2 depicts industries ranked by prevalence of noise exposure. Industries with the highest prevalences of workers with a history of noise exposure were the same industries as those with prevalent noise exposures, in nearly identical order and with higher prevalences: Mining (66%); combined Mining and OGE (65%); Construction (52%); Manufacturing (51%); Warehousing (47%); and AFFH (44%). Noise exposure and a history of noise exposure were lowest in Finance and Insurance (3% and 14% respectively).

Occupations with the highest prevalences of noise exposures were: Production (48%); Construction and Extraction (44%); Installation, Maintenance, and Repair (42%); Farming, Fishing, and Forestry (31%); Transportation and Material Moving (29%); and Protective Service (25%) (Table 3). The same six occupations had the highest prevalences of workers with a history of noise exposure, in a slightly different rank order and with higher prevalences than noise exposure. Noise exposure and a history of noise exposure were lowest in Computer and Mathematical (3%), and Community and Social Service (11%), respectively.

Seven percent of workers were exposed to both noise and ototoxic chemicals (Table 1). Males (11%) had more than five times the prevalence of exposure than females (2%), and exposures were highest in the 26–35 age group (9%). All races/ethnicities had prevalences ranging from 6–8% except for Asian workers (2%). Exposure prevalences were highest among those with less than a high school diploma (11%) and those with a high school diploma or GED (11%). Exposure was higher among obese workers (8%) than non-obese workers (6%) and was highest among current smokers (15%).

The industries with the highest combined exposures were the same industries with high prevalences of separate exposures to noise or ototoxic chemicals: Mining (37%); combined Mining and OGE (33%); Construction (19%); Manufacturing (17%); AFFH (17%); and Transportation (14%) (Table 2). Combined exposures were lowest in Health Care and Social Assistance (1%).

Eleven percent of workers reported hearing difficulty (Table 1). The prevalence of hearing difficulty was higher in males, increased substantially with age, and was highest among White workers (15%) followed by Other Race/Ethnicity (10%). The prevalence of hearing difficulty was fairly similar across levels of education, with the exception of workers with less than a high school diploma, who had a lower prevalence. The prevalence was higher among obese workers and among former smokers and current smokers.

Industries with the highest prevalences were: combined Mining and OGE (18%); Public Administration (15%); Warehousing (14%); AFFH (14%); Mining (14%); and Construction (13%) (Table 2). Hearing difficulty was lowest in Real Estate and Rental and Leasing (7%). The adjusted risk (PR) of having hearing difficulty was significantly higher in Warehousing (94% higher) and Public Administration (42% higher) than in the reference industry. The adjusted risk was significantly lower (40%) in Real Estate and Rental and Leasing than in the reference industry.

Occupations with the highest prevalences of hearing difficulty were: Protective Service (16%); Construction and Extraction (14%); Production (13%); Legal (13%); Community and Social Service (13%); and Arts, Design, Entertainment, Sports, and Media (13%) (Table 3). Hearing difficulty was lowest in Food Preparation and Serving Related (9%). Protective Service, Educational Instruction and Library, Construction and Extraction, and Production had adjusted risks 62%, 39%, 38% and 30% significantly higher, respectively, than the reference industry.

Thirty-seven percent of hearing difficulty cases among workers exposed to ototoxic chemicals were attributable to that exposure, with 1.4 million excess cases among exposed workers (Table 4). For workers exposed to secondhand smoke, 32% of their hearing difficulty cases were attributable to secondhand smoke, with 453,392 excess cases among the exposed. Among workers exposed to noise, 43% of their hearing difficulty cases were attributable to noise, with 2.1 million excess cases among the exposed that could be prevented if the noise was reduced to a safe level. Fifty percent of hearing difficulty cases among workers with a history of noise exposure were attributable to that history, with 4.5 million excess cases among the exposed that could be prevented. Among workers with combined exposures to noise and ototoxic chemicals, 41% of their hearing difficulty cases were attributable to these exposures, with nearly 1 million excess cases among the exposed.

This is the first study to estimate the overall prevalence of U.S. worker exposure to ototoxic chemicals, across substance classes, and by industry, occupation and demographic using a nationally representative sample. It is also the first study to estimate the prevalence of combined exposures to noise and ototoxic chemicals. In addition, this is the first study to provide the prevalence of workers with noise exposure in the past 12 months and with a history of noise exposure, side-by-side, along with ototoxicant exposures. This better elucidated those industries and occupations with multiple exposures that can damage hearing so they can be targeted for intervention. Secondhand smoke exposure was also provided separately so that it can be examined relative to other health outcomes. Lastly, this study provided the AF for each of the exposures to identify the number of excess cases among the exposed, and for the noise-exposed, the number of preventable cases of hearing difficulty if the noise exposure was reduced to a safe level.

This study had limitations. The chemicals that workers were asked about are not an exhaustive list of known ototoxic chemicals. This would make the question unwieldy, and there are also chemicals that are suspected to be ototoxic, but more research is needed to confirm their ototoxic properties. Therefore, it is likely that the ‘true’ prevalence of ototoxic chemical exposure is higher than reported in this study. As this is a cross-sectional study, no causation can be inferred between exposures and the outcome. However, causation is already well-established elsewhere between noise and hearing difficulty, and for some of the ototoxic chemicals.^2,3,5

All survey responses were self-reported, which can introduce bias and inaccuracy. However, most of the NHIS questions have been validated over time. The hearing difficulty question was validated against audiometric threshold estimates,⁶⁴ although research indicates that individuals sometimes under-report hearing difficulty, especially if it is mild or affecting the higher frequencies.^65–67 Recall of occupational noise exposure has been found to be valid.^68–70 While the ototoxic chemical exposure questions are new, the secondhand smoke question is very similar to a prior NHIS question and was modified for application to ototoxicity and consistency between the two ototoxic chemical questions. The prior secondhand smoke question was included regularly in NHIS supplements and analyses have been published.^12,71 The ototoxicant question was designed to cover the most common and important ototoxic chemical exposures in terminology that workers could likely recognize and understand.^3,5 There is precedent for using self-report to assess chemical exposures. Examples include the National Health and Nutrition Examination Survey (NHANES) including question(s) every year since 1999 about pesticide exposures, and during 1999–2000 about skin conditions and specific chemical exposures.⁷² The U.S. Department of Labor Occupational Information Network (O*NET) has included a question about the frequency of chemical exposures⁷³ and the Hispanic Community Health Study – Survey of Latinos (HCHS-SOL) includes chemical exposure questions on their occupation survey.⁷⁴

Due to a lack of sample size for OGE, no estimates could be provided. OGE does have some workers at high-risk for hearing loss.⁷⁵ Estimates were provided combining OGE with Mining in order to achieve some representation for this industry. The NHIS underwent a significant redesign in 2019, including changes to weighting to address nonresponse, which could affect some comparisons to prior findings using pre-2019 NHIS data.⁷⁶ Finally, SOC and NAICS are economic classifications which are not designed to group workers with similar exposures together.

Hearing hazards in the workplace are common. This study was the first to report the prevalence of ototoxic chemical exposure among U.S. workers. 22 million workers are exposed to ototoxic chemicals each year. NIOSH, OSHA, AIHA, ACGIH and ACOEM have all recognized the importance of this contributor to worker hearing loss.^3,17,53–57 This exposure cannot be overlooked when protecting worker hearing. Both noise and ototoxic chemical exposures can be reduced using the hierarchy of controls. In addition, ACGIH highly recommends annual audiograms for workers with exposures ≥20% of the threshold limit value for certain ototoxicants.⁵³ The U.S. Army and Air Force also enroll their soldiers and airmen in hearing conservation programs when ototoxicant exposures exceed 50% of the occupational exposure limit.^58,59 The 12 million workers with both noise and ototoxic chemical exposures deserve special attention in hearing conservation programs. More surveillance would also be needed to produce estimates for industries for which there was insufficient sample size.