Air Pollution And Asthma: Traffic's Deadly Impact

Traffic-related air pollution (TRAP) is a significant public health concern, with potential links to the development of asthma in both children and adults. While the causal relationship between air pollution and asthma is not yet fully understood, accumulating evidence suggests that TRAP exposure may contribute to the onset and exacerbation of asthma symptoms. The impact of TRAP is particularly pronounced in urban areas, with a higher risk for individuals residing near major roadways or traffic arteries. Certain demographic groups, such as individuals from low-income communities, communities of color, and pregnant women, are more vulnerable to the health effects of air pollution, including asthma development and exacerbation. Addressing TRAP and its potential consequences on respiratory health has become a crucial focus for governments and organizations aiming to mitigate the burden of asthma and improve overall public health.

Traffic-related air pollution (TRAP) and asthma in children

There is a growing body of evidence to suggest that traffic-related air pollution (TRAP) is linked to asthma in children. Several studies have found a positive association between TRAP exposure and the development of asthma in children, with one study of ten European cities attributing 14% of childhood asthma cases to near-road TRAP.

TRAP exposure has been found to enhance the effects of ambient exposure to PM2.5, nitrogen oxides, and carbon monoxide, which are known to have adverse effects on respiratory health. The impact of TRAP is particularly prominent in urban areas, with a study of two communities in Southern California estimating that reducing traffic-related nitrogen dioxide and ozone to background levels could reduce bronchitic episodes in asthmatic children by 36-70%.

The exact mechanisms by which TRAP exposure contributes to asthma are not yet fully understood. However, recent research suggests that air pollutants may suppress genes that regulate the immune system's ability to differentiate allergens from dangerous foreign substances. This can lead to an inappropriate inflammatory response, even to substances that are not harmful, which can trigger asthma attacks.

African American children have been identified as being particularly vulnerable to the effects of air pollution, with low levels of outdoor ozone associated with respiratory changes and adverse outcomes in those with difficult-to-treat asthma. Additionally, communities of color are often concentrated in more polluted places, leading to higher rates of emergency department visits for asthma. Low-income communities also tend to be located closer to sources of pollution and have fewer resources to relocate, making them more susceptible to the health impacts of TRAP.

To mitigate the impact of TRAP on asthma in children, various interventions have been suggested. These include community-level interventions such as urban planning to create "smart" cities with more green spaces and separate walking and cycling paths away from motorized streets. Governments also have a role in monitoring air pollution, informing the population about risks, and implementing measures to control the release of pollutants, such as promoting alternative fuels and fuel-cleaning options.

TRAP and adult-onset asthma

While the impact of air pollution on asthma is well-documented, the causal link between traffic-related air pollution (TRAP) and adult-onset asthma is less clear. However, there is some evidence to suggest a correlation.

Several studies have found an association between TRAP and the development of childhood asthma, with a particular focus on the impact of early exposure to TRAP on lung development. This has been linked to the development of both transient and persistent asthma/wheezing phenotypes in children. However, the relationship between TRAP exposure and the development of different asthma phenotypes is not yet fully understood, and the significance of these associations is inconsistent across studies.

In terms of adult-onset asthma, a meta-analysis of six cohorts in the European Study of Cohorts for Air Pollution Effects (ESCAPE) found that exposure to higher levels of nitrogen dioxide (NO2) was associated with an increased incidence of adult-onset asthma. However, the results were not statistically significant. Another study in Southern California estimated that reducing traffic-related NO2 and ozone levels could reduce bronchitic episodes in asthmatics by 36-70%. Additionally, a study of ten European cities found that 15% of all exacerbations of childhood asthma were attributed to TRAP exposure, suggesting that TRAP may also play a role in adult asthma exacerbations.

While the exact mechanisms are not yet fully understood, it is believed that TRAP may contribute to the development and persistence of asthma through epigenetic changes. For example, exposure to NO2 has been linked to differential methylation and expression of genes involved in antioxidant defense pathways.

Overall, while the evidence suggests a correlation between TRAP and adult-onset asthma, more research is needed to establish a clear causal relationship. Further studies should focus on assessing the impact of strategies aimed at reducing TRAP exposure and the development of new cases of asthma in both children and adults.

TRAP exposure and exacerbation of asthma symptoms

Exposure to traffic-related air pollution (TRAP) has been linked to the development, persistence, and exacerbation of asthma. TRAP comprises a mixture of vehicle exhausts, secondary pollutants, evaporative emissions from vehicles, and non-combustion emissions (e.g., road dust). It is a significant concern, especially in urban areas, where a large proportion of the global population resides.

Several studies have found a positive association between TRAP exposure and asthma symptoms. The phase 3 ISAAC study, involving over 500,000 children and adolescents across five continents, identified a dose-response relationship between asthma symptoms and self-reported exposure to truck traffic. Similarly, the Southern California Children's Health Study showed that children living in communities with higher levels of nitrogen dioxide (NO2), PM10, and PM2.5 had increased respiratory symptoms, including chronic lower respiratory issues, phlegm production, bronchitis, and wheezing. These studies highlight the detrimental effects of TRAP on respiratory health, particularly in vulnerable populations such as children.

The impact of TRAP on asthma exacerbation is further supported by epidemiological research. A systematic review of worldwide traffic emissions and health science by the Health Effects Institute (HEI) concluded that TRAP exposure causes asthma exacerbation in children. Additionally, short-term exposure to outdoor air pollution, including TRAP, has been linked to worsening asthma symptoms, leading to an increased number of emergency room visits and hospital admissions. This association is particularly pronounced in children, who are more susceptible to the effects of outdoor air pollution.

The mechanisms underlying the link between TRAP exposure and asthma exacerbation involve the oxidative potential of certain pollutants. For example, NO2 and O3, which are present in TRAP, can lead to oxidative stress and enhance allergic responses and airway inflammation. Diesel exhaust particles (DEP), a significant component of TRAP, have been shown to induce oxidative stress, airway hyper-responsiveness, and inflammation. These processes can trigger asthma attacks and worsen respiratory symptoms in individuals with pre-existing asthma.

To mitigate the impact of TRAP on asthma exacerbation, various interventions have been proposed. These include community-level interventions such as urban planning with more green spaces and the development of separate walking and cycling paths away from motorized streets. Additionally, governments can play a crucial role by monitoring air pollution levels, educating the population about risks, and implementing measures to control the release of pollutants, such as promoting alternative fuels and fuel-cleaning options. Individual-level interventions, such as the use of air cleaners and improved household ventilation, can also help reduce the negative impact of TRAP on asthma symptoms.

TRAP and asthma in vulnerable groups

While the evidence base linking air pollution to asthma exacerbations has been accumulating for decades, recent studies also suggest that air pollution may cause new-onset asthma. Traffic-Related Air Pollution (TRAP) is considered the main source of urban air pollution. Vehicle emissions, including ozone, particulate matter, carbon oxide, nitrogen oxide, and hydrocarbons, are all part of TRAP and have been linked to the development of asthma.

Children are particularly vulnerable to TRAP as their respiratory and immune systems are immature, and they have larger unit weight ventilation and more outdoor activities than adults. In a study of ten European cities, 14% of new asthma cases in children were attributed to TRAP exposure. A study in Los Angeles County attributed 8% of new asthma cases to traffic proximity.

Several studies have identified modifiers of the association between early postnatal TRAP exposure and asthma in adolescence. Those with a parental history of allergies, especially asthma, early stressful family events, and lower respiratory tract infections (LRTI) appear to be more vulnerable to TRAP. The PARIS birth cohort study found that these sub-groups of children were more vulnerable to TRAP regarding asthma prevalence at 4 years old and reduced lung function at 8-9 years old.

TRAP-induced asthma is a distinct phenotype of asthma, characterised by increased levels of serum IL-17A in children and increased CD4+IL13+IL17+ double-producing T effector memory cells in mice. Exposure to TRAP has been implicated not only in the development of asthma but also in its persistence and exacerbation.

TRAP and asthma: interventions and prevention

Asthma is a major global health problem affecting around 235 million people worldwide. Traffic-related air pollution (TRAP) has been implicated in asthma development, persistence, and exacerbation. TRAP exposure is highly significant as large segments of the global population reside in zones that are most impacted by TRAP, and schools are often located in high TRAP exposure areas.

Interventions and Prevention

• The use of air cleaners, low-polluting sources for cooking and heating, or personal devices to monitor indoor air pollution can help reduce the negative impact of indoor air pollution on asthma. However, these solutions may be less accessible to people in developing countries.
• Governments can promote health initiatives such as smoking cessation programs, avoidance of secondhand smoke (SHS), education on improving household ventilation, and mold removal. These measures are cost-effective and can be implemented worldwide, regardless of socioeconomic status.
• Community-level interventions such as urban planning for "smart" cities with more green spaces located away from major traffic arteries and industrial areas can help reduce respiratory morbidity.
• Governments should monitor air pollution, inform the population about the risks during high air pollution levels, and take measures to control the release of particulate matter (PM). This includes considering alternative fuels, fuel-cleaning options, and alternative production processes and technologies.
• The development of walking and cycling paths separated from motorized streets can also help reduce exposure to TRAP.
• The World Health Organization (WHO) and other organizations provide guidelines and recommendations for asthma management. The Global Initiative for Asthma (GINA) guidelines, for example, suggest a stepwise and control-based approach to treatment, including assessment, adjustment of treatment, and review of responses to minimize symptom burden and the risk of exacerbations.
• Anti-inflammatory medications, such as inhaled corticosteroids (ICS), are the mainstay in the treatment of asthma, while short-acting beta-agonists (SABAs) are used as quick-relief or rescue medicines to rapidly ease acute symptoms.
• The ENRIECO project in Europe is a collaboration of multiple birth cohort studies that will provide further information on early exposures related to asthma incidence.
• New methodologies for the quantification of TRAP will enable the accurate assessment of exposure, allowing for the design and implementation of early interventions for vulnerable populations.

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