Traffic Pollution: Asthma Trigger?

Traffic-related air pollution (TRAP) has been associated with adverse cardiorespiratory effects, including asthma. People living near busy roadways are exposed to harmful pollutants, including nitrogen oxides, particle pollution, and VOCs, which can cause oxidative injury to the airways, leading to inflammation, remodelling, and an increased risk of sensitisation. Several studies have found a positive association between exposure to air pollution and the development of asthma, with vulnerable groups such as children and people of colour being disproportionately affected. While not all studies support a causal link, the weight of evidence suggests that traffic pollution can contribute to both new-onset asthma and the exacerbation of pre-existing asthma.

Traffic pollution and asthma in children

Asthma is one of the most common chronic airway diseases among children in the United States. Several studies have found a link between traffic-related air pollution and the onset of childhood asthma. Traffic and power generation are the main sources of urban air pollution, and people who live closest to highways and busy roads are most likely to be affected.

A study of ten European cities found that 14% of childhood asthma cases and 15% of exacerbations of childhood asthma were attributed to exposure to pollutants related to road traffic. Another study of two communities in Southern California estimated that reducing traffic-related nitrogen dioxide and ozone to background levels would reduce bronchitic episodes in asthmatics by 36-70%.

The phase 3 ISAAC study, which included over 500,000 children and adolescents across five continents, found a dose-response association between asthma symptoms and self-reported exposure to truck traffic. Several other studies have also found associations between traffic-related air pollution and childhood asthma. For example, a study of two German birth cohorts found associations between PM2.5 light absorption, roadway proximity, and asthmatic bronchitis.

Additionally, prenatal and postnatal exposure to air pollutants, particularly PM2.5, has been linked to the development of asthma in children. A birth cohort study in Taiwan found that both prenatal and postnatal exposure to air pollutants were associated with a higher risk of developing asthma. Another population-based birth cohort study found a positive association between perinatal exposure to air pollution and asthma incidence during preschool years.

To reduce the impact of traffic pollution on childhood asthma, several measures have been recommended. These include personal strategies such as using N95 facemasks when pollution levels are high, shifting to active travel like walking or cycling, and selecting low-traffic routes. Community-level interventions, such as urban planning with more green spaces and separate walking and cycling paths, can also help reduce respiratory morbidity.

Air pollution and asthma exacerbation

Air pollution has been linked to asthma exacerbation for several decades, with studies suggesting a contribution to new-onset asthma as well. Traffic and power generation are the main sources of urban air pollution. Outdoor air pollution contributed to over 3% of the annual disability-adjusted life years lost in the 2010 Global Burden of Disease comparative risk assessment. This is a notable increase from the 2000 estimate.

Several observational studies have suggested that outdoor air pollution may induce or aggravate asthma. However, the results are inconclusive due to the presence of numerous moderators that influence this association. A study of ten European cities found that 14% of the cases of incident asthma in children and 15% of all exacerbations of childhood asthma were attributed to exposure to pollutants related to road traffic. Urbanization is an important contributor to asthma, and with many urban centres in the developing world undergoing rapid population growth accompanied by increased outdoor air pollution, the global burden of asthma is likely to increase.

The phase 3 ISAAC study, representing over 500,000 children and adolescents across five continents, identified a dose-response association between symptoms of asthma and self-reported exposure to truck traffic. The effects of short-term ambient exposure to PM2·5, nitrogen oxides, and carbon monoxide were increased by exposure to higher than the median modelled traffic exposure. This shows the importance of considering both regional air pollution and long-term TRAP exposure in studies of health effects. A study of two communities in Southern California estimated that reducing traffic-related nitrogen dioxide and ozone to background levels would reduce bronchitic episodes in asthmatics by 36–70%.

Several risk reduction measures have been recommended, including personal strategies, community and government interventions, and the use of effect modifiers. Educating patients with asthma to minimise their exposure to air pollutants is an important step in asthma management. Some measures that can be beneficial include the use of close-fitting N95 facemasks when air pollution levels are high, shifting from motorized to active travel (e.g., cycling, walking), selecting low-traffic routes or those with open spaces, driving with windows closed, and maintaining car air filtration systems and internal circulation. Alerts on the occurrence of peaks of pollution, freely available online for the general population, can also help people with asthma avoid outdoor activities during periods of poor air quality.

Air pollution and new-onset asthma

Air pollution has been linked to an increased risk of new-onset asthma, particularly in children. Several studies have found a positive association between exposure to air pollutants, especially during pregnancy and early childhood, and the subsequent development of asthma. This is especially true in urban areas, where traffic-related air pollution (TRAP) is a significant contributor to poor air quality.

TRAP is a complex mixture of harmful pollutants, including nitrogen oxides, particle pollution, and volatile organic compounds (VOCs) from vehicles' tailpipes, as well as dust from roadsides and the abrasion of the road surface itself. People living near busy roadways are at a higher risk of exposure to these pollutants, and the impact decreases with distance from major roads. High-volume traffic on secondary roads, with frequent stops and accelerations, has also been associated with asthma symptoms.

The impact of TRAP on asthma is evident in studies from various regions. A study of ten European cities attributed 14% of childhood asthma cases to exposure to road traffic pollutants. In the United States, TRAP accounted for 27-42% of childhood asthma cases in 2000 and 18-36% in 2010, with higher percentages in urban areas and among lower-income groups. The phase 3 ISAAC study, involving over 500,000 children and adolescents across five continents, also identified a link between self-reported exposure to truck traffic and asthma symptoms.

While the evidence suggests a strong association between air pollution and new-onset asthma, the specific causal mechanisms are still being investigated. Air pollutants are believed to cause oxidative injury to the airways, leading to inflammation, remodelling, and an increased risk of sensitisation. Long-term exposure to air pollution, particularly in early life, appears to be a key factor contributing to the development of asthma. However, it is important to note that not all studies have found a definitive causal link, and the strength of the evidence varies.

Air pollution and asthma in pregnancy

Air pollution is a mixture of particulate matter, gases, and air toxics. It can occur both outdoors and indoors. Outdoor air pollution is mostly caused by traffic and power generation, while indoor air pollution is often caused by toxins such as cigarette smoke.

Pregnant women, children, babies, the elderly, and those with respiratory problems are the most vulnerable to the harmful effects of air pollution. Exposure to air pollution during pregnancy can have negative consequences for both the mother and the developing baby. It has been linked to an increased risk of preterm birth, low birth weight, stillbirth, and congenital abnormalities. Additionally, research has found that exposure to air pollution during pregnancy increases the baby's chances of developing asthma later in life.

Several studies have examined the effects of air pollution on pregnancy outcomes. A study by the Stockholm Environment Institute (SEI) at the University of York found that nearly three million babies are born prematurely each year due to air pollution. Another study in Allegheny County, PA, showed that exposure to air pollution in the first trimester increased the risk of preeclampsia and high blood pressure, which can lead to early birth, low birth weight, and preterm labor. The AQRH study also found that exposure to VOCs (volatile organic compounds) doubled the risk of preeclampsia in pregnant women with asthma compared to those without asthma.

Furthermore, air pollution has been linked to long-term respiratory issues, such as asthma and allergies. A study of ten European cities attributed 14% of childhood asthma cases and 15% of asthma exacerbations to exposure to road traffic pollutants. Another study in the United States estimated that reducing traffic-related nitrogen dioxide and ozone levels could decrease bronchitic episodes in asthmatics by 36-70%.

To reduce the risk of adverse pregnancy outcomes and respiratory issues, it is essential to minimize exposure to air pollutants. This can be achieved through personal strategies, community interventions, and government actions. Personal strategies include the use of N95 facemasks, choosing active travel over motorized transport, selecting low-traffic routes, and staying informed about local air pollution levels. Community interventions may involve urban planning with more green spaces and separate walking and cycling paths. Governments can play a role in monitoring air pollution, informing the public about risks, and implementing measures to control the release of pollutants.

Air pollution and asthma in vulnerable groups

Air pollution is a contamination of the air with substances that harm human health or the environment. It can include gases, chemicals, or small particles in the air. Outdoor air pollution comes from both manmade and natural sources, while indoor air pollution can come from various sources, such as allergens and volatile organic compounds (VOCs).

Traffic-related air pollution (TRAP) is a significant contributor to outdoor air pollution, particularly in urban areas. TRAP includes pollutants such as nitrogen dioxide (NO2), particulate matter (PM), and ozone (O3). Exposure to these pollutants has been linked to an increased risk of developing asthma and exacerbating existing asthma symptoms. People with pre-existing asthma are more vulnerable to the effects of air pollution, as it can irritate the airways and trigger asthma attacks.

Children are especially vulnerable to the harmful effects of air pollution, and the estimated six million children in the United States with asthma are at particular risk. This may be because younger children traditionally spend more time outdoors and their respiratory systems are still developing. African American adolescents have been found to be more vulnerable to air pollution than other children, with low levels of outdoor ozone associated with respiratory changes and other health outcomes.

In addition to TRAP, indoor air pollution can also impact asthma in vulnerable groups. For example, exposure to indoor molds has been found to improve asthma symptoms and reduce medication use. Therefore, it is important to consider both outdoor and indoor air pollution when addressing the impact of air pollution on asthma in vulnerable groups.

To reduce the impact of air pollution on asthma in vulnerable groups, various risk reduction measures have been recommended, including personal strategies, community and government interventions, and the use of effect modifiers. Educating patients with asthma to minimize their exposure to air pollutants is an important step in asthma management. This can include using close-fitting N95 facemasks when pollution levels are high, choosing active travel over motorized transport, selecting low-traffic routes, and staying informed about local air pollution levels.

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