Occupational hearing loss (OHL) is the most common work-related illness in the United States [NIOSH, 2013a]. It most often results from chronic exposure to hazardous noise (≥85 dBA) but can be caused by a single instantaneous high noise exposure or exposure to ototoxic chemicals [Nelson et al., 2005]. Approximately 22 million U.S. workers are exposed to hazardous noise at work [Tak et al., 2009]. Noise regulations exist in most industries to prevent or ameliorate hazardous occupational noise. However, noise exposure limits and mandated prevention efforts vary by industry [NIOSH, 1998]. Some industries, such as agriculture, have no noise regulation at all [Suter, 2003]. Hearing loss is permanent, potentially debilitating, and affects workers both at home and on the job [Hetu et al., 1995; Morata et al., 2005; Seidman and Standring, 2010]. However, OHL resulting from noise and ototoxic chemical exposures is entirely preventable [Themann et al., 2013a,b].

The level of hearing loss deemed acceptable in establishing U.S. noise regulations is based on preserving hearing for conversational speech, defined by the National Institute for Occupational Safety and Health (NIOSH) as an average hearing level of 25 dBor less (better) at the frequencies 1,000, 2,000, 3,000, and 4,000 Hertz (Hz). When hearing ability exceeds the 25 dB average, a worker is said to have developed a “material hearing impairment.” No known studies have examined whether current regulations are effectively preventing material hearing impairment among U.S. workers and few studies have examined trends in worker hearing loss.

Daniell et al. [2002] reported that workers’ compensation claims for hearing loss doubled between 1984 and 1991 and multiplied by twelve by 1998 in Washington State. They postulated that the large increase was likely due to reporting phenomena. McCall and Horwitz [2004] reported that workers’ compensation claims decreased in Oregon between 1984 and 1998 following improvements in state occupational safety and health standards.

Tak and Calvert [2008] examined the overall trend in the prevalence of self-reported hearing difficulty among workers during 1997–2003 and found that the prevalence slowly decreased until 2000, but then fluctuated thereafter. Bureau of Labor Statistics (BLS) data has indicated a gradual reduction in the incidence of Occupational Safety and Health Administration (OSHA) standard threshold shifts in hearing during 2004–2007 [Hager, 2006, 2007, 2008, 2009]. However, BLS estimates must be interpreted with caution, in part since certain types of employers are not represented [Leigh and Miller, 1998], and economic incentives may encourage under-reporting [Leigh and Miller, 1998; Azaroff et al., 2002].

NIOSH established the OHL Surveillance Project in 2009 to address the lack of a national surveillance program for OHL. Through partnerships with audiometric testing service providers and others, hereafter referred to as providers, NIOSH collects de-identified worker audiograms originally completed for regulatory compliance purposes, including historical audiograms.

The purpose of this study was to estimate the incidence and prevalence of material hearing impairment, hereafter referred to as hearing loss, for noise-exposed U.S. workers by industry sector and time period using NIOSH OHL Surveillance Project data. The risk of incident hearing loss as compared with a reference time period was also estimated. No previous studies have examined 30 years of hearing loss prevalence, incidence and risk by industry sector.

The results of worker audiograms were used to identify hearing loss. Audiometric records included date of birth, gender, threshold values at frequencies 500, 1,000, 2,000, 3,000, 4,000, 6,000, and 8,000 Hz, and North American Industry Classification System (NAICS) codes [U.S. Department of Commerce, The Kraus Organization Limited, 2007; U.S. Census Bureau, 2011]. NAICS codes range from two-digit to six-digit numbers and industry specificity increases with each digit. Date of hire and occupation were not available for most cases. Education, race, income, smoking status, noise and ototoxic chemical exposure information were also not available.

Sample demographics by time period are provided in Table II. Within all time periods, most workers were males (77–82%) and the largest percentage were employed in the U.S. Midwest (39–49%). Over time, the percentages of female workers and older workers increased. Although the number of audiograms available in our sample increased dramatically over time, the proportions for industry sectors remained fairly constant. The manufacturing sector accounted for the majority of the audiograms (68–76%).

Table III includes the estimated prevalence of workers with hearing loss by industry sector over time. The overall prevalence of hearing loss for all industry sectors combined remained very consistent over time, with 20% in Period 1 and 19% in Period 6. The prevalence also remained consistent within most industry sectors over time, with the exception of (i) Agriculture, Forestry, Fishing and Hunting, which decreased from 33% to 14%; (ii) Transportation, Warehousing and Utilities, which decreased from 18% to 12%; and (iii) Healthcare and Social Assistance, which increased from 12% to 18%. The prevalence within the Mining sector stayed consistent and high (24–27% in Periods 3–6). The prevalence of hearing loss was also high in the Construction sector; above 24% in four of the six time periods. Figure 1 depicts these prevalence estimates in line graph format.

Hearing loss incidence estimates are provided in Table IV, and depicted graphically in Figure 2. The overall incidence of hearing loss for workers in all industry sectors slowly decreased over time, from 9% in Period 2 to 7% in Period 6, and this trend was observed within most industry sectors. Most values hovered around 7–9%. The incidence in several sectors increased in Period 5, dramatically so within Construction and Healthcare and Social Assistance, followed by a reduction. After an increase in Period 3, there was a dramatic drop in incidence within the Agriculture, Forestry, Fishing and Hunting sector. The Construction (9%) and Mining (8%) industry sectors had the highest incidences in Period 6, while Transportation, Warehousing and Utilities had the lowest (5%).

Table V includes adjusted risk estimates for incident hearing loss as compared with a reference time period. These risk estimates are depicted graphically in Figure 3, with the exception of the Mining and Healthcare and Social Assistance sectors, which have different reference groups. The risk of hearing loss for all industry sectors combined consistently decreased over time, from PR = 0.80 (CI = 0.77–0.83) in Period 3 to PR = 0.54 (CI = 0.52–0.55) in Period 6, which was significantly lower than the risk of incident hearing loss in the reference time period. Risks generally decreased over time within industry sectors. However, there was a statistically significant interaction between industry sector and time period (P < 0.05), indicating different patterns of hearing loss risk among the sectors.

Nearly all of the industry sectors had PRs in Period 6 that were significantly lower than the reference time period. However, the Healthcare and Social Assistance sector had a risk increase in Period 5 and slight decrease in Period 6; neither risk estimate significantly different than the risk in the reference period (Period 4). While the risks in this sector appeared to be higher than the other sectors, we had used a later reference time period. In a subsequent analysis in which Period 4 was designated as the reference group for all industry sectors, the risks for workers in Healthcare and Social Assistance were still higher than the other sectors, with the exception of Construction (data not shown). The risk of incident hearing loss in the Mining sector in Period 6 was not significantly lower than the reference time period (Period 5). Lastly, after an increase in Period 3, there was a dramatic drop in the risk within the Agriculture, Forestry, Fishing and Hunting industry sector.

Our study results indicated that overall, the prevalence of hearing loss remained fairly constant at 20% over the last 30 years. However, the incidence and adjusted risk of incident hearing loss steadily decreased, albeit slowly, for most industry sectors.

Transportation, Warehousing and Utilities had a lower prevalence of hearing loss than any other industry sector, and a low incidence and risk. This seemed counter-intuitive since some transportation workers, such as railroad workers, have been found to have a very high prevalence and risk of hearing difficulty [Tak and Calvert, 2008]. This sector groups NAICS codes 22, 48, and 49, which include workers with likely vastly different levels of noise exposure. In our sample, 65% of the workers in this sector were from the Couriers and Messengers industry (NAICS 492). The prevalence in this industry is among the lowest in the NIOSH data repository (8%) and we have used it as a reference industry in other analyses [Masterson et al., 2013]. The predominance of presumed low-exposed workers likely reduced the overall prevalence. There was also a 16% increase in the proportion of women working in this sector, and fewer women experience hearing loss than men [Palmer et al., 2001], likely due to a variety of reasons including differences in exposures [Themann et al., 2013a].

It is unclear why the Agriculture, Forestry, Fishing and Hunting sector had such a sharp drop in the prevalence and incidence of hearing loss. This sector has unique challenges in preventing OHL, which include a lack of noise regulation for agricultural workers and numerous barriers to regular audiometric testing. Recent papers indicate a moderate prevalence of hearing loss in this sector overall (15–16%), but also fairly high adjusted risks [Tak and Calvert, 2008; Masterson et al., 2013]. There was a 21% increase in the proportion of women in this sector from Periods 1 to 6, the largest increase of any sector. More research is needed in this sector.

The Mining and Construction sectors had the highest prevalence and incidence of hearing loss. These findings are consistent with the results of other studies [Tak and Calvert, 2008; Engdahl and Tambs, 2010; Masterson et al., 2013]. The Mining sector has proportionally more noise-exposed workers than any other U.S. industry [Tak et al., 2009], and the Construction sector has less stringent hearing conservation requirements than most industries. The mobile, seasonal nature of construction work and large proportion of independent contractors also contributes to the difficulty in implementing hearing conservation practices.

Healthcare and Social Assistance had the third highest incidence and one of the highest risks. This sector is not usually associated with a higher risk of hearing loss, and the prevalence has been estimated to be 9–10% overall [Tak and Calvert, 2008; Masterson et al., 2013]. However, other research [Masterson, 2012] has suggested that this sector has a higher prevalence of shifts in hearing. Shifts in hearing can be early indicators of hearing loss and are a measure of the effectiveness of hearing conservation programs [NIOSH, 1998]. Only a small proportion of this sector is exposed to hazardous noise (~3.5%) and it has been found that the self-reported use of hearing protection among noise-exposed workers is directly related to the prevalence of noise exposure in that industry [Tak et al., 2009]. Tak et al. [2009] observed that 74% of the noise-exposed workers in Healthcare and Social Assistance reported not wearing hearing protection. A culture of hearing loss prevention may be less likely to develop within industries with small numbers of noise-exposed workers due to a lack of awareness, experience or resources. Increased interventions in these industries may be warranted.

The risk assessment reported in the NIOSH recommended standard for occupational noise exposure [NIOSH, 1998] estimated that 25% of workers exposed to daily occupational noise levels of 90 dBA (the current exposure limit in most industries) would develop a material hearing impairment after a 40-year working lifetime. Because NIOSH considered 25% to be too high, it proposed an exposure limit that would protect more workers [NIOSH, 1998]. Our prevalence estimate of 20% approaches the 25% mark, but many of the workers in our sample do not have 40 years of noise-exposed work experience, suggesting that enhanced regulatory and preventive efforts may be needed.

Fortunately, our results also indicate that progress is being made. Although the prevalence has remained fairly constant over time, the incidence and risk of incident hearing loss has decreased in most industry sectors over the last 30 years. Hoffman et al. [2010] examined nationally-representative population data and reported that the prevalence of hearing impairment decreased significantly from the periods 1959–1962 to 1999–2004 for the general population. Reduction in occupational exposures, or improved hearing conservation efforts are possible explanations or contributors to the reduced prevalence observed by Hoffman and the reduced incidence observed in our study. However, other factors may be influencing these results, such as improved treatment of middle ear disorders, and the overall reduction in smoking prevalence, another risk factor for hearing loss [Agrawal et al., 2009].

This study had limitations. The data were a convenience sample from providers who agreed to share their data with NIOSH, and our sample may not be representative of all noise-exposed workers, especially within industries like Construction where audiometric testing is not required. However, estimates from other studies utilizing random samples have yielded mostly similar results [Tak and Calvert, 2008; Helmkamp et al., 2013]. Hearing loss can be determined from an audiogram but the work-relatedness of the loss can only be inferred in the absence of additional information. To strengthen this inference, audiograms with attributes unlikely to be related to OHL were excluded. The quality level of the audiometric data may have varied by provider and providers also “inherited” audiograms. In some cases, the industry coding was performed by the provider, with the potential for inconsistencies. When estimating prevalence, we examined one audiogram per worker without a “confirmation” audiogram. It is possible that a small number of hearing losses were temporary shifts in hearing. However, temporary threshold shift may be a sign that a worker is over-exposed to noise and can be a precursor of permanent hearing loss. All or nearly all the workers in the sample were noise-exposed workers, including workers in our reference time period. The reference period (1986–1990) was proximate to the 1983 OSHA Noise Standard amendment (29 CFR 1910.95), and pre-dated both the 2000 update to the Mining Noise Standard (30 CFR 62) and the 2002 Noise Standard for Construction (29 CFR 26.52). There is some evidence that regulations are protective [Verbeek et al., 2009]. As such, workers in our reference time period were assumed to be at higher risk than workers in later time periods. Most of the PRs are <1, indicating that the risk is lower now than in the reference time period. The PR values are not stand-alone estimates of risk. Finally, NAICS is an economic classification system which may not group workers with similar exposures together, and industries could only be examined by sector grouping due to sample size limitations.

This is the first known study to estimate and compare the prevalence and incidence of worker hearing loss by industry sector, and over such an expanded time period. Rather than relying on self-reported hearing ability, we examined audiograms from workers employed at thousands of U.S. companies. The sample size allowed us to exclude audiograms with negative slope, improving the accuracy. We also excluded audiograms of poor quality or depicting characteristics likely due to non-occupational exposures. With the exception of age group (slightly fewer younger workers were eliminated), the demographics of these excluded audiograms and the study sample were very similar, indicating no gender, geographical region, provider or industry sector was disproportionately removed from the study sample (data not shown).

This study elucidates the trends in the burden and risk of hearing loss among workers in hazardous noise environments. Efforts to reduce both the burden and risk are still needed. Special efforts should be targeted at the Mining, Construction, and Healthcare and Social Assistance industry sectors. Despite progress over the past three decades, OHL remains a problem in the U.S.