Does outdoor air pollution induce new cases of asthma? Biological plausibility and evidence; a review

Abstract

It is widely accepted that air pollution can exacerbate asthma in those who already have the condition. What is less clear is whether air pollution can contribute to the initiation of new cases of asthma. Mechanistic evidence from toxicological studies, together with recent information on genes that predispose towards the development of asthma, suggests that this is biologically plausible, particularly in the light of the current understanding of asthma as a complex disease with a variety of phenotypes. The epidemiological evidence for associations between ambient levels of air pollutants and asthma prevalence at a whole community level is unconvincing; meta-analysis confirms a lack of association. In contrast, a meta-analysis of cohort studies found an association between asthma incidence and within-community variations in air pollution (largely traffic dominated). Similarly, a systematic review suggests an association of asthma prevalence with exposure to traffic, although only in those living very close to heavily trafficked roads carrying a lot of trucks. Based on this evidence, the UK's Committee on the Medical Effects of Air Pollutants recently concluded that, overall, the evidence is consistent with the possibility that outdoor air pollution might play a role in causing asthma in susceptible individuals living very close to busy roads carrying a lot of truck traffic. Nonetheless, the effect on public health is unlikely to be large: air pollutants are likely to make only a small contribution, compared with other factors, in the development of asthma, and in only a small proportion of the population.

It is not time to lower the guard!

“…Other studies, however, did not find any association, which could be due to differences in study design in terms of case definition and exposure assessment, populations, and periods of survey [5,6]. Overall, our colleagues in the UK suggested that the evidence is consistent with the possibility that outdoor air pollution might play a role in causing asthma in susceptible individuals living very close to busy roads carrying heavy lorry traffic [7], which is supported by increasing evidence in Europe and elsewhere. Notably, the observed odds ratios were not elevated (unlike the risk of lung cancer from smoking tobacco, for example), which can be explained by the fact that asthma is a multifactorial disease and is dependent on the simultaneous occurrence of several exposures to both genetic and environmental factors [8].…”

A rapid assessment of the impact of hazard reduction burning around Sydney, May 2016

“…COMEAP recently concluded that the epidemiological evidence for associations between ambient levels of air pollutants and asthma prevalence at a whole community level was unconvincing; a meta-analysis confirmed a lack of association (Gowers et al, 2012). In contrast, a meta-analysis of cohort studies found an association between asthma incidence and within-community variations in air pollution (largely traffic dominated).…”

Early-Life Air Pollution and Asthma Risk in Minority Children. The GALA II and SAGE II Studies

“…A similar meta-analysis with 19 studies conducted exclusively in children found a statistically significant association between the incidence of childhood asthma and NO 2 (OR, 1.14; 95% CI, 1.06-1.24, per 10-mg/m 3 increase) (16). A review of these and other studies concluded that traffic-related pollution may play a role in the development of asthma, especially in individuals living near highvolume roadways (25).…”