Misuse of Pollution Reference Standards: No Safe Level of Air Pollution

The burden of ill health owing to exposure to air pollution is well established and widely accepted (1). Despite this, effective regulation of exposure to air pollution has lagged in many countries. The World Health Organization’s updated guidelines (2) have recently been published, many years after the previous version. They are considerably strengthened and welcome. A key recommendation of these guidelines is that “the accumulated evidence is sufficient to justify actions to reduce population exposure to key air pollutants…” (2). This has important implications for policy and regulatory practice.

In this issue of the Journal, Wei and colleagues (pp. 1075–1083) highlight the limitations of the existing regulatory approach, at least in relation to protection from severe exacerbations of asthma requiring hospital admission (3). Increases in ambient concentrations of NO₂ and particulate matter ⩽2.5 μm in aerodynamic diameter (fine particulate matter) were associated with an increased risk of hospitalization for asthma over the succeeding 6-day period. For fine particulate matter, the effect was greatest on the day of the high pollutant exposure and the succeeding 3 days and then began to decay. For NO₂, the effect was greatest on the 4 days after the exposure event. In general, the effect was robust to differences in individual-level characteristics, except that it was stronger in people who have only had a single admission to hospital than those with multiple admissions. Some characteristics of communities made them more susceptible to the adverse effects of both pollutants: low population density, higher average body mass index, greater distance to the nearest hospital, and greater neighborhood degrees of disadvantage. An important finding was that effect sizes (that is, the magnitude of the increased risk of hospitalization for a unit increase in pollutant exposure) were greatest when the population being analyzed was limited to those with exposures well below the existing reference standard (National Ambient Air Quality Standards). The strengths of this study include its focus on vulnerable populations, those enrolled in Medicaid, the high spatial and temporal resolution of air pollutant exposure measurements, and the sophisticated adjustment for, and stratification by, both individual- and area-level covariates. These methodological strengths, together with the broad range of exposures, add value to the findings.

The finding that health effects were greatest at concentrations below the existing reference standard has two important implications: First, from a mechanistic standpoint, the slope of the exposure–response relationship is steeper at lower degrees of exposure; and second, from a policy standpoint, there are substantial benefits in reducing exposure to ambient pollutants, and in preventing increases in exposure, even at concentrations that are well below reference standards or proposed thresholds.

The present study (3) joins with several others that have failed to demonstrate a concentration below which adverse health effects of ambient pollutants do not occur. Indeed, steeper slopes at lower concentrations of fine particulates have also been demonstrated for daily mortality (4) and for cardiovascular hospitalizations (5). The all-cause mortality risk associated with long-term exposure to both fine particulates and NO₂ has been demonstrated in relatively low-range exposure settings, such as Australia (6), without evidence of a lower threshold and with evidence of steeper slope at lower concentrations. Finally, the effect of both lifetime cumulative exposure and current exposure to NO₂ on the risk of having current asthma among primary school children exists, in Australia, within a range of exposures well below usual reference standards (7). Hence, there is strong empirical evidence that there is no “safe” amount of air pollution below which adverse effects do not occur.

The finding that those living in disadvantaged neighborhoods were at greatest risk of experiencing more asthma hospitalizations when exposed to higher amounts of air pollution is important. It accords with evidence of a similar interaction in relation to adverse cardiovascular outcomes (8, 9) and represents an effect that is over and above the association between disadvantage and the risk of higher air pollutant exposures. The mechanism for this association remains unproven, but its existence is clear.

The policy implications of these findings are critical: there is no safe amount of air pollution (10). Treating reference standards as a license to pollute up to those concentrations and a free pass to allow continued emissions below those concentrations cannot be accepted based on the current evidence. Transport, industry, and planning decisions need to be based on the need to exert continuing downward pressure on emissions of fine particulates and NO₂ and the need to avoid any increases in these emissions, particularly where vulnerable populations may be exposed. Concern about vulnerable populations, particularly those who are disadvantaged but also the very young and the very old and those with preexisting health conditions, have important implications for decisions on the siting of sensitive facilities (e.g., childcare centers, schools, aged care facilities, and hospitals). Among other factors, planning should consider the need to minimize exposure to pollutants, not simply ensure they are below arbitrary threshold standards.

The challenge is immense, because anthropogenic sources of emissions contributing to ambient fine particulate and NO₂ are ubiquitous in most societies: transport, agriculture, energy, mining, and construction sectors all play a role. The good news is that there are cobenefits shared between the actions required for carbon pollution reduction, to combat global warming, and those required to ensure clean, safe air. As a global scientific and health-focused community, we need to ensure that we are in the vanguard of advocacy for achieving these benefits.