摘要：Objectives. We measured noise levels associated with various forms of mass transit and compared them to exposure guidelines designed to protect against noise-induced hearing loss. Methods. We used noise dosimetry to measure time-integrated noise levels in a representative sample of New York City mass transit systems (subways, buses, ferries, tramway, and commuter railways) aboard transit vehicles and at vehicle boarding platforms or terminals during June and July 2007. Results. Of the transit types evaluated, subway cars and platforms had the highest associated equivalent continuous average (Leq) and maximum noise levels. All transit types had Leq levels appreciably above 70 A-weighted decibels, the threshold at which noise-induced hearing loss is considered possible. Conclusions. Mass transit noise exposure has the potential to exceed limits recommended by the World Health Organization and the US Environmental Protection Agency and thus cause noise-induced hearing loss among riders of all forms of mass transit given sufficient exposure durations. Environmental noise–control efforts in mass transit and, in cases in which controls are infeasible, the use of personal hearing protection would benefit the ridership's hearing health. For the first time in history, more than half of the world's population lives in cities, and it is projected that more than two thirds of the population will live in cities by 2030. 1 An important factor supporting the growth and viability of urban centers is mass transportation, which is rapidly expanding to keep pace with increasing demand. For example, in 2004 there were 95 subway systems worldwide; today there are 167, a 76% increase in only 5 years. 2 Although there are well-documented environmental and public health benefits associated with mass transit, interest in the health and safety effects of mass transit on urban communities is increasing. 3 – 5 A particular concern is the potential for mass transit to result in excessive exposure to noise. Noise exposure is a function of 2 main factors: (1) the frequency-weighted exposure level, measured in A-weighted decibels (dBA), and (2) the exposure duration. The causal association between chronic exposure to excessive noise and permanent, irreversible, noise-induced hearing loss (NIHL) is well known, as are the adverse social, psychological, and occupational effects associated with the condition. Nonauditory adverse health effects have also been reported, 6 – 8 and recent research suggests that excessive noise exposure may be linked to hypertension and ischemic heart disease, disruptions in stress hormones, and sleep disorders. 9 – 12 There are no comprehensive national or international surveillance programs for hearing loss. Worldwide, more than 250 million people are estimated to suffer from hearing loss, of which at least 30 million cases represent NIHL. 13 In the United States alone, between 3 to 10 million people are estimated to have NIHL. 14 Hearing loss from all causes ranks among the top 10 most common serious health problems worldwide, and NIHL is the leading occupational disease in industrialized nations. 14 , 15 The limited data available suggest not only that NIHL prevalence and incidence rates are extraordinarily high but also that the associated costs are enormous. 16 , 17 Importantly, even though US occupational exposure regulations have been in place for decades, rates of NIHL-related workers' compensation cases remain high. Therefore, nonoccupational sources of exposure are coming under scrutiny, including mass transit. The size of the population exposed to mass transit noise is of considerable magnitude. The US mass transit network, with an infrastructure encompassing subways, buses, commuter and light rail, ferry boats, trolleys, and tramways, is the largest in the world, with 9.7 billion passenger rides in 2006. 18 There are 14 subway systems in the United States, with a combined daily ridership in excess of 10 million people. 19 – 21 Five of the US systems are more than 75 years old, and the largest, the New York City subway system, with over 4 million riders per weekday, 22 is more than 100 years old. These older systems were designed before noise-control technologies were available. Worldwide, there are 2 subway systems with even greater ridership rates: Tokyo's is the largest at 2.6 billion passenger rides per year, and Moscow's is the second largest with 2.5 billion. 23 , 24 In a recent sound-level pilot survey on subways, 3 we noted levels that potentially exceeded the community exposure limits initially recommended by the US Environmental Protection Agency (EPA) in 1974 and confirmed by the World Health Organization (WHO) in 1998. WHO and EPA recommended daily allowable exposure times are 24 hours at 70 dBA, 8 hours at 75 dBA, 2.7 hours at 80 dBA, 0.9 hours at 85 dBA, and 0.3 hours at 90 dBA. Chronic exposures that exceed these allowable combinations of duration and noise level are expected to produce NIHL in some members of the exposed population. 25 , 26 The amount of NIHL anticipated to result from specific noise-exposure levels can be predicted with a model published by the International Organization for Standardization. 27 This model allows users to estimate the amount of NIHL expected to result from chronic 8-hour equivalent continuous average (Leq) noise exposures between 75 and 100 dBA or 24-hour Leq exposures between 70 and 95 dBA. The model permits the estimation of median values of expected NIHL as well as values for the 0.05 to 0.95 fractiles among an exposed population for given exposure levels and durations. Based on the WHO and EPA recommendations, chronic exposure to 80.3 dBA for more than 160 minutes per day may be expected to produce hearing loss in some exposed individuals, and a 90.2-dBA level likewise may cause hearing loss with just 18 minutes of exposure per day. Few data involving dosimetry measurements of noise exposures associated with mass transit have been reported previously. In a study of the daily noise exposures experienced by 32 people in Madrid, Spain, Diaz et al. 28 measured noise levels associated with a variety of self-reported transportation exposures with noise dosimeters. Zheng et al. 29 conducted 24-hour noise dosimetry on 221 residents of Beijing, China, and assessed the noise levels associated with self-reported activities, including commuting. Nearly all other studies that have evaluated noise levels associated with subway equipment are decades old and based on sound level measurements rather than dosimetry. In 1931, Stanton conducted an unpublished noise-level survey of the New York City subways, 30 and in 1971, Harris and Aitken 31 reported levels measured on specific New York City train line platforms and cars. A small sound level survey on a subway system in India was also recently reported. 32 Our current study expanded on our pilot study of subway noise and assessed average noise levels on a variety of types of mass transit to further evaluate noise exposure among transit riders.
