The relationship between transport noise and health outcomes is complex, in part because of the large number of factors involved as well as the range of health impacts, both direct and indirect. To enable the reader to come to grips with the complexity, we have divided the health outcomes into groups: those that are more directly linked to transport noise exposure and those that are more indirectly linked. Four health outcomes, namely annoyance, cognitive disruption, sleep problems, and noise-induced hearing loss, can be directly attributable to transport noise exposure. Less direct outcomes are stress, mental health, metabolic health, cardiovascular health, and overall health-related quality of life. Stress may occur as a direct response to noise, or may occur in response to the aforementioned direct effects. The stress response is a survival mechanism in the short term, but in the long term, stress may lead to systemic health conditions, namely metabolic and cardiovascular outcomes, and to mental health conditions. Finally, a global health outcome that incorporates all of the more direct outcomes is health-related quality of life. Other exposures associated with transport noise that may explain parts of the health outcomes need to be acknowledged, including exposure to social inequities, air pollution, and vibration. These may all be more likely to be experienced by people who are exposed to transport noise in the community and may thus influence the outcomes. Finally, transport noise appears to have more impact on health in those who are noise sensitive, thus noise sensitivity is a key moderator of all the effects observed.
1. Introduction
Transport is an important component of human society, integral to connectedness and economic prosperity. A by-product of transport is the creation of noise. The effects of noise on exposed communities range from direct psychophysiological responses to noise exposure (annoyance, cognitive disruption, sleep problems, and noise-induced hearing loss), to less direct effects such as mental health issues and physical health problems. The mechanisms are complex and involve interactions between the primary noise exposure and higher-level reactions; for example, stress can be a response to noise, but may also occur indirectly from reduced or poor sleep, or as a result of noise-induced hearing loss.
Several reviews have been published (e.g. Basner et al., Citation2014) including a set of eleven articles about the WHO European environmental noise guidelines (WHO Environmental noise guidelines for the European region, Citation2017). This article presents a more recent review and is intended for a wider audience based on a process model developed to elucidate the complex and interrelated nature of exposure to noise and impacts.
Noise exposure, including that from transport, does not occur in isolation from other factors influencing health. Those experiencing negative social inequities, such as poverty, poorer nutrition, or lower levels of education may be more likely to be living in places close to sources of transport noise; therefore, apparent health effects of noise may actually reflect the impact of social inequity. Furthermore, transport produces both air pollution and ground vibration which also impact health and well-being separately.
Figure 1 presents a process diagram of the health impacts of transport noise on which the structure of this paper is based, beginning with consideration of social inequities, air pollution, and vibration as concomitants of environmental exposure to transport noise. The moderating factors are then addressed, followed by discussion of the key health impacts starting with the more direct and then those that are more indirect.
Figure 1. Process diagram for the health impacts of transport noise.
For clarity, the review was structured around a process diagram (Figure 1), and aimed to be accessible to readers with little prior background in the area. Space is therefore dedicated to explaining the underlying principles as well as describing research in each area. The literature review is narrative in style, and authors were assigned topics within their areas of expertise as well as contributing overall. The search and criteria were therefore subjective.
2. Associated exposures
Transport noise is likely to be associated with other exposures that impact health, including fumes from internal combustion engines, and vibration caused either directly by heavy vehicles, or indirectly, as a by-product of high-level noise.
2.1. Air pollution
Vehicles produce air pollution including carbon monoxide, hydrocarbons and nitrogen oxides as well as particulate matter, presenting a health risk when inhaled (World Health Organization, Citation2014). Findings of direct associations between road traffic noise level and air pollution are mixed. For example, a significant correlation was found between nitrogen dioxide as a surrogate for traffic air pollution and noise measured at fixed sites (Foraster et al., Citation2009), whereas little correlation was found between air pollution levels and noise from road traffic in mobile measures from cyclists (Gelb & Apparicio, Citation2021).
Since exposure to both noise and air pollution impact negatively on health, the two need to be disentangled. In this respect, two studies are of note. The first is a study involving school-aged children (Clark et al., Citation2012) in which it was found that air pollution did not moderate the relationship between road traffic noise exposure, and cognitive or mental health measures. The second was a study that measured both annoyance and HRQoL that found no interaction between air pollution annoyance and noise annoyance, implying that air pollution and noise impact health independently (Shepherd et al., Citation2016). This lack of research has led to appeals for further research (Tzivian et al., Citation2015).
2.2. Vibration
At a physical level, audible sound is an acoustic wave by which mechanical energy is propagated through the atmosphere. Acoustic waves can also produce sensations of vibration that are felt rather than heard. In the context of transportation noise, the majority of transport modes produce ground-borne (i.e. rail, road) or airborne (i.e. aviation) vibrations that may interact with audible noise to amplify health effects. For example, low frequency aircraft noise can displace structures and their contents, and produce greater annoyance responses in houses bereft of acoustic insulation (Fidell et al., Citation2002). Typically, vibration is not considered as a separate impact from noise, and more research is required to disentangle the effects of vibration and noise on annoyance (Anciaes et al., Citation2017) and other health outcomes.
In terms of the cohorts affected by vibration, research has largely focused upon its impact on transport users, such as passengers or vehicle controllers, while neglecting those passively exposed to transport vibration, such as those living in communities located close to noise sources. In the case of railway noise, in a laboratory-based study, Maigrot et al. (Citation2020) suggested that the presence of vibration does not amplify annoyance to noise, a finding replicated in a population health study on passenger trains (Maclachlan et al., Citation2018). However, this study did not consider freight trains. Ogren et al. (Citation2017) estimated that rail-related vibration velocities of 0.48 and 0.98 mm/s resulted in 20% and 40% annoyance responses, respectively. This is consistent with the findings of Woodcock et al. (Citation2016) suggesting that annoyance increases if the vibration-induced rattle is audible. Di et al. (Citation2019) reported that perceived vibration moderates the relationship between noise exposure and noise annoyance, more for trains than road traffic, because trains produce more vibration even at equal sound pressure levels (Yokoshima et al., Citation2021). Finally, vibration from freight trains is linked to sleep disturbance, which in turn may mediate the relationship between train noise and annoyance (Smith et al., Citation2016).