Climate Change and Respiratory Health

Current Evidence and Knowledge Gaps

Tim K Takaro; Kim Knowlton; John R Balmes


Expert Rev Resp Med. 2013;7(4):349-361. 

In This Article

Key Knowledge & Data Gaps

Gaps in Epidemiology

A substantial body of epidemiological research describes how rising atmospheric temperatures and alterations in weather patterns from climate change, are linked to changes in the distribution of respiratory disease and respiratory-related mortality and morbidity.[7,8,30,42,96–99]

Changes in the frequency, intensity, and areal extent of extreme weather events, including heat waves, drought, extreme precipitation, and coastal flooding, are exacerbated by climate change and they in turn affect the distribution and incidence of respiratory diseases. There are, however, remaining challenges in attributing specific changes in the patterns of disease incidence to the local and regional effects of climate change, and more accurate predictive models are needed.[100] There are disparities in the temporal and spatial scales at which air pollution monitoring and health tracking data are collected, administrative barriers to sharing and linking environmental-health data sets, and a general paucity of such monitoring data (even in the USA) that might otherwise allow questions about changing trends in prevalence of climate-sensitive respiratory conditions to be assessed in the recent past or approaching near-real time. Residential housing characteristics, air-conditioning availability and use, individual activity patterns, and individual and community-level temperature and air pollutant exposures all factor into a thorough assessment of climate-health effects.[69,101,102]

Other questions to be investigated concern the degree to which populations may adapt over time to ozone exposures, or if mortality displacement may occur, similar to heat responses.[103] This refers to a short-term increase in mortality rates, often in response to temperature or air pollution events, which can be followed in subsequent weeks by a decrease in overall mortality. The exposure effectively advances or "displaces" the date of death to an earlier day for those most vulnerable in a population, leading to fewer deaths after the event. There have been relatively few assessments thus far of the proximal and distal effects of wildfire smoke on respiratory health. Monitoring networks are needed that can link satellite data of smoke plumes to near-ground monitoring of emissions, ambient downwind concentrations, and associated health effects, in order to provide a foundation for creating early-warning systems in areas downwind of fires where respiratory health could be threatened.

Beyond laboratory and field studies of how rising temperatures and atmospheric CO2 can influence the timing, production, and allergenicity of pollen,[23,31] questions remain about allergy/asthma-relevant threshold levels of pollen concentrations in the air, how those concentrations may vary locally, and what local modifying effects co-exposures to other climate change-sensitive air pollutants like ground-level ozone may have. Furthermore, do local carbon dioxide sources have local effects on pollen production? Our ability to address these research questions will be enhanced by establishing a more finely-resolved national network of daily pollen monitoring sites, linked at comparable temporal and spatial scales to reporting networks for near real-time health tracking of allergy and asthma health effects, linked with carbon dioxide emission source data, and other health-relevant air pollutant monitoring.

A better understanding is needed of the precise biophysical mechanisms underlying heat waves impacts on respiratory mortality;[42] the effects of increasing minimum nighttime temperatures on internal biophysical set-points, relative to cardiovascular, respiratory or other morbidity thresholds; and the degree to which increasing temperature variability associated with a changing climate could increase both respiratory morbidity and mortality in the future.[36,102]

Gaps in Exposure Assessment

Air pollutants and their precursors can affect climate, and, in turn, the distributions of air pollutants are highly dependent upon regional climate.[78] When considering strategies to abate air pollution and mitigate anthropogenic climate warming, policymakers face tradeoffs and synergies. For example, sulfate is a major component of PM2.5 pollution in many regions, but reducing sulfate for health reasons could lead to a rapid rise in surface temperatures as the atmospheric cooling effect of sulfates diminishes.[104] In the absence of emission changes, a warming climate may increase air pollution in many polluted regions, an impact that has been referred to as "a climate change penalty" on air quality.

A wide range of model estimates exists for regional air quality both at present and for future projections. Increasing the length of time over which relationships between relevant meteorological variables and air quality have been observed can provide useful information for evaluating models. Further study of the observed relationships may help to improve our understanding of the links between air quality and climate.

Gaps in mechanistic understanding limit confidence in projecting future air quality in a changing climate. For example, how ozone (O3) levels will be impacted by climate change is strongly dependent on how organic nitrates (RONO2) are considered,[37,104] and there is major variability in how models treat the impact of RONO2 on oxides of nitrogen (NOX) availability as an O3 sink.[105] Uncertainties remain about biogenic volatile organic compound oxidation and subsequent secondary aerosol formation.[106] Aerosol-oxidant interactions also require further study as they may determine particulate matter (PM) air quality in some regions.[107]

Human interactions with the biosphere are crucial to understand because vegetation acts as both a source and a sink for many air pollutants. The attribution of O3 and PM air pollution to "anthropogenic" versus "biogenic" sources is complicated by atmospheric chemistry that involves both anthropogenic and biogenic precursors and by land-use changes, which alter biogenic sources.[108] Sources from agriculture and livestock sectors are generally difficult to model, but non-negligible, particularly for methane and ammonia (NH3). Human-driven changes in land-use and land cover, such as urbanization or shifts between forests and agriculture, could dramatically alter future O3 and aerosol precursor emissions.[78]

Climate-driven changes in PM could be large, but there are major uncertainties in the model projections.[78] Aerosolized particulate concentrations are particularly sensitive to precipitation changes and are expected to decrease in regions with increased rainfall. Large contributions are possible from "natural" aerosol sources, such as carbonaceous aerosols from wildfires, mineral dust, and biogenic precursors to secondary organic aerosol. As noted earlier, in regions experiencing a warmer and dryer climate, wildfires are expected to increase. Seasonal dust storms will likely increase and lead to hazardous levels of PM2.5 in regions downwind of major desert source regions, and in some cases, leading to long-range transport across oceans.

Future PM levels will be a function of changes in both emissions and climate. Concentrations of PM are driven by local as well as regional anthropogenic emissions, depend on regional oxidant levels, and are complex to model. Changes in sulfate aerosol concentrations, however, generally follow changes in sulfur dioxide (SO2) emissions. Changes in NOX emissions influence nitrate aerosols to a lesser extent than the SO2-sulfate relationship due to competition between sulfate and nitrate for ammonium, such that nitrate aerosol is inversely dependent on sulfate. Continued reductions in SO2 emissions alongside rising NH3 emission could lead to nitrate aerosol levels equivalent to or larger than sulfate aerosol levels in some regions over the 21st century.[109]

The impact of anthropogenic aerosol on clouds is more uncertain. There is an indirect cooling effect of aerosol, also known as the cloud albedo effect.[110] Very little is known on the contribution of different aerosol components to this effect. While several studies have tried to quantify the cloud albedo effect of BC-containing particles, the sign of the cloud effect (i.e., cooling or warming) is model-dependent and varies with the BC to OC mass ratio, the size of emitted particles, and the magnitude of the emission change.[111]

Other significant gaps in our understanding of human health risk in a changing climate are also of interest to pulmonologists. How do heat stress and air pollution interact in different geographical and social settings? What makes communities resilient to extreme heat and air pollution events? Can vulnerable populations (e.g., old, young, impoverished, with chronic cardio-respiratory conditions) be protected? What factors determine a community's adaptive capacity and can this capacity be 'grown' by efforts through the Green Climate Fund?