Traffic Pollution: Highways Vs. Urban Streets

Motor vehicles are a significant source of urban air pollution, and living near roads is linked to adverse health effects. While PM2.5 levels are only slightly elevated near roads, pollutants like ultrafine particles, carbon monoxide, NO2, black carbon, polycyclic aromatic hydrocarbons, and metals are more concentrated in these areas. The impact of traffic emissions extends up to 300-500 meters from highways or major roads, with metals associated with brake and tire wear, such as copper, iron, and zinc, being more prevalent near roadways than in urban areas. Studies have also shown the negative health impacts of prolonged exposure to traffic pollution, including cardiovascular issues and abnormal reproductive health outcomes. These findings highlight the health risks associated with living in close proximity to roads and the potential consequences for residents near highways or urban streets.

Air pollution and health

Air pollution is a serious issue that poses significant risks to human health. Research has shown that air pollution can trigger heart attacks and strokes and has also been linked to autism spectrum disorder. One of the main sources of air pollution is motor vehicles, with pollutants such as ultrafine particles, carbon monoxide, NO2, black carbon, and polycyclic aromatic hydrocarbons being elevated near roads. These pollutants have been found to have adverse effects on health, with studies showing decreases in high-frequency heart rate variability and increases in low-frequency components in volunteers with type 2 diabetes who were exposed to traffic pollution during car rides.

The impact of air pollution on health is not limited to those who live near highways or busy roads. The pollution can travel farther than expected, especially during nighttime and early morning hours when stagnant weather conditions trap freeway pollution near the ground, allowing it to drift more than a mile downwind. Additionally, certain areas may be more affected by pollution due to geographical factors, such as canyons or street canyons with tall buildings that trap pollution.

Proximity to roads does play a significant role in exposure to air pollution. Studies have identified an exposure zone within a range of up to 300-500 meters from a highway or major road as the area most affected by traffic emissions. However, the impact of pollution extends beyond this zone, especially for specific pollutants. For example, levels of ultrafine particles were found to decrease rapidly in the first 150 meters but then level off, indicating that areas farther from the road can still experience elevated levels of certain pollutants.

Socioeconomic factors also come into play, with low-income communities experiencing higher exposures to emissions from busy highways. This inequality in exposure to air pollution can further exacerbate health disparities between different socioeconomic groups. Therefore, it is essential to consider not only the distance from roadways but also the broader environmental and socioeconomic context when assessing the health impacts of air pollution.

To mitigate the health risks associated with air pollution, individuals can take precautions such as avoiding living near highway interchanges, freeway ramps, and major intersections, and areas with tall buildings that trap pollution. Vertical separation from freeways and the presence of open spaces can help disperse pollutants. Additionally, postponing outdoor exercise until later in the morning can help reduce exposure to spikes in traffic pollution that occur during pre-sunrise hours.

Near-roadway pollution

Several studies have found higher levels of pollutants in close proximity to roads. The Health Effects Institute Panel identified an exposure zone within a range of up to 300–500 meters from a highway or major road as the area most affected by traffic emissions. The specific pollutants elevated near roads include ultrafine particles, carbon monoxide, NO2, black carbon, polycyclic aromatic hydrocarbons, and some metals. These pollutants have been linked to adverse health effects, with one study finding an association between PM2.5 road dust and mortality.

Zhu et al. (2002) found that ultrafine particles, black carbon, and total PM counts decreased rapidly in the first 150 meters from a roadway and then levelled off. Similarly, Roorda-Knape et al. (1998) and Gilbert et al. (2003) found that concentrations of pollutants like black smoke, PM2.5, NO2, and benzene decreased to background levels within 100–200 meters of a roadway. These studies highlight the existence of a concentration gradient for pollutants, with higher concentrations near roadways.

The impact of near-roadway pollution is a significant concern, especially for vulnerable populations such as children, older adults, and people with pre-existing cardiopulmonary conditions. The EPA is actively researching and implementing strategies to mitigate the effects of near-roadway pollution, including the use of roadside vegetation barriers and noise barriers.

While EPA's emissions and fuel standards have helped reduce pollutant emissions from vehicles, additional research is needed to develop further mitigation strategies. This includes understanding the health effects of single and multiple pollutants and the specific components of vehicle emissions associated with these effects. The EPA also supports independent organizations like the Health Effects Institute, which provides scientific research on the health impacts of motor vehicle pollutants.

Socioeconomic factors

Several socioeconomic factors contribute to and are impacted by air pollution. Firstly, there is a clear link between socioeconomic status and exposure to air pollution, with low-income communities facing higher exposure to air pollutants and other environmental hazards. This is often due to the proximity of pollution sources to disadvantaged communities. For example, people of color are among the groups most vulnerable to air pollution, and they are more likely to reside in counties with higher pollution levels, which is a consequence of residential segregation.

Research indicates that income and socioeconomic status play a significant role in the unequal distribution of air pollution exposure. A study of New Jersey residents found that communities with larger African American populations, lower home values, and lower median incomes faced a higher risk of premature death from long-term exposure to particle pollution. Similarly, a study of Atlanta, Georgia, revealed that high-poverty zip codes had increased rates of asthma attacks attributed to particle pollution.

The triple jeopardy hypothesis highlights the health disparities faced by low socioeconomic status (SES) communities. Firstly, they experience higher exposure to air pollutants; secondly, they are more susceptible to poor health due to psychosocial stressors, discrimination, and limited access to health-promoting resources; and thirdly, these factors result in health disparities driven by environmental factors.

Socioeconomic inequalities also influence the impact of air pollution on different racial and ethnic groups. For instance, a study found that Hispanics, Asians, and especially blacks, had a higher risk of premature death from particle pollution than whites, even when income was not the differentiating factor. Chronic stress resulting from discrimination and other psychosocial factors may contribute to these disparities.

Additionally, indoor air pollution within homes may also vary across socioeconomic groups. Building characteristics, such as build quality, volume, and ventilation, can influence indoor air quality. Disadvantaged communities may lack the resources to ensure optimal indoor air quality, further exacerbating the health inequalities caused by outdoor air pollution. Overall, these socioeconomic factors play a critical role in understanding the unequal distribution of air pollution and its impacts on vulnerable communities.

Vertical separation from highways

Several studies have found that living near highways or major roads can expose individuals to higher levels of pollutants and adverse health effects. The Health Effects Institute Panel identified an exposure zone within a range of up to 300–500 meters from a highway as the area most affected by traffic emissions. Ultrafine particles, black carbon, and carbon monoxide levels exhibit a sharp decline within the first 100 to 150 meters of roadways, but other pollutants like NO2, metals, and PM2.5 can remain elevated closer to roads.

The benefits of vertical separation are evident in areas like West Oakland, where rerouting the Cypress Freeway and constructing the Mandela Parkway at street level resulted in significant pollution reductions. Annual average pollutant concentrations within 25 meters of Mandela Parkway decreased by 66% for NOx and 48% for BC due to the rerouting.

Additionally, natural obstacles such as sound walls, trees, and plants can also provide some protection from freeway pollution. While not designed to block vehicle emissions entirely, these barriers can help disperse pollutants and reduce their impact on nearby residents.

Overall, vertical separation from highways, combined with strategic freeway rerouting and the presence of natural obstacles, can help mitigate the pollution exposure experienced by individuals living or working nearby. These measures can contribute to improving air quality and reducing health risks for communities located near busy roadways.

Pollution from intersections and ramps

Several studies have found that proximity to roads is associated with higher levels of pollutants. The specific pollutants that are more elevated near roads include ultrafine particles, carbon monoxide, NO2, black carbon, polycyclic aromatic hydrocarbons, and some metals. These pollutants have been linked to adverse health effects, including increased risk of cancer, asthma, heart disease, and other illnesses.

Intersections and highway ramps are particularly problematic when it comes to pollution. Vehicles stopping and accelerating at intersections and ramps emit large amounts of exhaust and toxic particles, such as copper dust. The pollution levels are further exacerbated by the presence of diesel trucks, which emit many times more harmful gases and particles than cars.

Regulators and scientists have identified highway interchanges and freeway ramps as hotspots for pollution, with residents in these areas exposed to twice as much pollution. Similarly, major intersections with stoplights can also be significant sources of pollution, as the constant stopping and starting of vehicles generates a "big cloud of fairly concentrated pollution."

The impact of pollution from intersections and ramps can be influenced by various factors. For example, wind patterns, freeway design, time of day, and the types of vehicles and buildings in the area can all affect the extent and reach of pollution. Additionally, living in a smoggy area or near freeways frequented by diesel trucks can result in a "double dose" of pollution from traffic emissions and regional pollution.

To protect themselves from the harmful effects of pollution, individuals are advised to consider not only their distance from freeways but also their specific surroundings and the time of day. For instance, traffic pollution drifts much farther during late-night and early-morning hours, extending more than a mile downwind from the freeway.

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