Subway Noise Pollution: Understanding The Causes And Impact

Subways are a significant source of noise pollution, which can have adverse effects on the health of residents living nearby and the millions of commuters who use them. Noise exposure and noise-induced hearing loss (NIHL) are significant problems, especially in urban settings in industrialized nations. In the US alone, it is believed that over 20-30 million Americans are exposed to excessive noise levels, with about 10 million suffering from NIHL. The noise generated by subways and their ventilation systems can cause or contribute to hearing loss, and the closed, sound-proof underground space of subway stations can amplify and prolong the effects of noise.

Subway systems are often old and affect hearing

The US has the largest mass transit infrastructure in the world, with 33 million riders each weekday. New York City alone has over 7 million riders. The New York City subway system is over 100 years old and is the largest in the US. Older systems like this were designed before noise-control technologies were available.

Noise exposure and noise-induced hearing loss (NIHL) are significant problems, especially in urban settings in industrialized nations. In the US, it is believed that over 20–30 million Americans are exposed to excessively high levels of noise, with about 10 million estimated to have NIHL. Determining the impact of risk factors for sociocusis (non-work-related hearing loss resulting from exposure to high levels of noise associated with recreational activities and transit use) on overall hearing levels is complex. The contribution of chronic exposure to short periods of high noise, as might be encountered on subways, especially older subway systems, to hearing loss is not entirely clear.

The seriousness of the outcomes of noise exposure is well documented. Transportation noise accounted for about 13% of participants' total noise exposure in one study, and for participants older than 60 years, transportation noise was the primary source of noise exposure. Another study found that the mean Leq noise level was 76.1 ±7.8 dBA, consistent with the measures for several of the transit types examined. The World Health Organization and the US Environmental Protection Agency have set limits to noise exposure to prevent noise-induced hearing loss. However, mass transit noise exposure has the potential to exceed these limits, causing noise-induced hearing loss among riders of all forms of mass transit given sufficient exposure durations.

The noise level in subway cars is typically around 75-85 decibels, but this can vary depending on factors such as the number of passengers and whether anyone is shouting or performing. When a train passes through a station, the noise level can be much higher, ranging from 80 decibels to over 100 decibels in some cases. These noise levels are comparable to the noise of a jet taking off or a power lawnmower. The noise in subway stations can be amplified due to the closed, sound-proof underground space, and it can be challenging to transmit this noise outside to the external environment.

To address the issue of noise pollution in older subway systems, comprehensive noise exposure assessments, audiometric testing, and statistical modelling can be employed to estimate the contributions of past noise exposure and identify risk factors for hearing loss. Additionally, rational space design and efficient ventilation systems can play a role in noise control and improving overall environmental health in subway stations.

Millions of passengers are exposed to noise pollution

The seriousness of the outcomes of noise exposure is well-documented. Noise exposure and noise-induced hearing loss (NIHL) are significant global problems, especially in urban settings in industrialized nations. In the U.S., over 20-30 million Americans are exposed to excessively high levels of noise, with about 10 million estimated to have NIHL. Worldwide, this number increases to over 200 million people. While most NIHL is believed to be due to occupational exposure, determining the impact of risk factors for sociocusis (non-work-related hearing loss resulting from exposure to high levels of noise associated with recreational activities and transit use) is complex. The contribution of chronic exposure to short periods of high noise, as encountered on subways, to hearing loss is not entirely clear. However, NIHL is incremental and involves a gradual diminution in hearing acuity.

The next step in evaluating exposure to subway-related noise among riders is personal dosimetry measurements, which would provide time-integrated estimates of noise exposure on subway platforms and cars. This would be followed by extensive assessments of noise exposure histories and audiometric testing or self-reported hearing status. Statistical models are available to estimate the contributions of past noise exposure history, including other sources of noise and the relative contributions of aging and other risk factors for hearing loss.

Additionally, noise inside a subway station is difficult to transmit outside to the external environment due to the closed, sound-proof underground space. This results in worse and longer-term effects, as the noise is amplified in the closed subway space and produces uncomfortable draft sensations that may affect passengers' integumentary, respiratory, and cardiovascular systems.

Ventilation systems can amplify noise in closed spaces

Ventilation systems are essential for maintaining indoor air quality and comfort, but they can also produce unwanted noise that affects people's well-being and productivity. This is especially true in closed spaces like subway stations, where noise is difficult to transmit outside and thus produces worse and longer-term effects.

The ducts are a major source of noise in ventilation systems, as they can transmit and amplify noise from fans or other components, as well as generate noise from airflow turbulence, friction, or expansion. The grilles and terminal units, which deliver air to occupied spaces, can also affect noise levels by creating airflow noise or radiating noise from ducts or fans. The diameter of the ducts and the number of bends can also impact the noise level.

To prevent or reduce duct noise, ducts should be insulated with sound-absorbing materials such as fiberglass, mineral wool, or foam, and any gaps or holes should be sealed to prevent air leakage and noise transmission. Sharp bends, abrupt transitions, or narrow openings should be avoided, as these can cause airflow disturbances and increase noise.

In addition to duct insulation and sealing, silencers can be installed at the inlet or outlet of the fan or along the ducts to absorb and dissipate sound energy, reducing noise levels. Proper installation and adjustment of the ventilation unit and its components are also crucial for minimizing noise.

Soundproofing the entire HVAC system or using soundproof blankets can also help reduce noise transmission through vents and into adjacent rooms. Regular cleaning of vents and ducts can also help cut down on noise caused by dirty components.

Poor ventilation systems can increase indoor air pollution

Secondly, poor ventilation systems can become contaminated and spread these pollutants throughout a building. For example, HVAC systems can spread mould in duct lining or bacteria on coils or filters. The HVAC duct distribution system can also spread pollutants from one area of a building to another.

Thirdly, poor ventilation systems may not adequately control temperature and humidity, which can increase concentrations of some pollutants and spur the growth of mould. In cool climates, inadequate ventilation in winter can contribute to excessive moisture and humidity, as normal activities like cooking and bathing create moisture, and there is insufficient ventilation to remove it.

Finally, poor ventilation systems can allow outdoor air pollutants to enter a building. For instance, if you live near a busy freeway, diesel emissions from trucks can enter your home, and outdoor particulates can be drawn inside when the heating or cooling system draws air into a home.

In the context of subways, poor ventilation systems can increase indoor air pollution in subway stations. The piston wind ventilation strategy used to exhaust heat can spread air pollutants throughout the entire station. The continuous operation and heavy load of subway ventilation systems make them more prone to pollution by particulate matter and microbes. The closed, underground space of subway stations also makes it difficult to transmit noise outside, which can have worse long-term effects.

Subway stations attract large crowds, adding to the noise

Subway stations, particularly in large cities, attract large crowds of people. This is a result of the high ridership of subways, with over 30 million mass transit subway riders in the US alone. In New York City, there are over 7 million riders. Such large crowds generate a lot of noise, contributing to the overall noise pollution in subway stations.

The noise generated by crowds in subway stations can have significant impacts on commuters' hearing. Studies have shown that even brief exposure to noise in subway stations can lead to temporary threshold shift (TTS) and elevated distortion product otoacoustic emissions (DPOAEs). These effects can be mitigated with the use of hearing protection, such as earplugs, which can significantly reduce PTA thresholds.

The impact of crowd noise on hearing may be influenced by various factors, including the design of the station. For example, curved stations tend to be much louder than straight stations, leading to higher noise exposure for commuters. Additionally, the number of trains passing through and the time of day can also affect noise levels.

Crowd noise in subway stations can also have indirect effects on health. Large crowds in enclosed spaces can increase indoor temperature, humidity, and carbon dioxide levels, promoting the growth of microbes and potentially leading to respiratory issues. Additionally, the combination of crowd-generated noise and train-induced wind can result in uncomfortable draft sensations and even skin, respiratory, or cardiovascular conditions.

To address the noise pollution caused by crowds in subway stations, several measures can be considered. These may include optimizing station design to minimize noise amplification, implementing noise barriers or absorptive materials, and improving ventilation systems to reduce the spread of air pollutants and manage noise transmission.

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