Fine particulate matter—also called PM2.5—is one of the most dangerous air pollutants. PM2.5 is composed of tiny solids and liquids floating in the air, and comes from a variety of sources such as motor vehicles, coal-fired power plants, and wildfires. Once inhaled, these particles can pass through the respiratory system, sneak into the blood and circulatory system, and cause serious health problems. Short-term exposure to PM2.5 has been consistently linked with multiple causes of hospital admissions, emergency room visits, and mortality. Of these causes, the link between PM2.5 and cardiovascular and respiratory diseases have been studied extensively. Additionally, exposure to PM2.5 has been linked to preterm birth, diabetes, and neurological disorders. 

In many cases, however, air pollution epidemiological studies are driven by a “plausible hypothesis” that a specific disease could be affected by exposure. This limits us to known potential mechanisms, leaving unanswered whether other diseases are associated with the exposure and, if so, to what extent.

To better understand the health effects of short-term exposure to PM2.5, we conducted a comprehensive analysis to investigate associations between short-term PM2.5 exposure and risks of hospital admissions for 214 disease groups, using 95 million hospital admission records among Medicare fee-for-service beneficiaries ≥65 years of age in the United States. In addition to confirming associations for several already known diseases, we also discovered several previously unknown causes of hospital admissions that are positively associated with short-term exposure to PM2.5. These include multiple prevalent causes among older adults such as fluid and electrolyte disorders, septicemia, anemia, urinary tract infections, and renal failure. Importantly, most of these identified hospitalization risks occurred when daily PM2.5 level was below the current WHO air quality guideline.

We also evaluated the associated economic costs of short-term exposure to PM2.5. Among the Medicare fee-for-service beneficiaries, each 1 µg·m-3 increase in short-term exposure to PM2.5 was associated with an annual increase of 5,693 hospitalizations, 32,315 days of hospitalization, and 634 deaths at discharge, corresponding to $100 million US dollars in inpatient and post-acute costs, and $6.5 billion US dollars in value of statistical life (i.e., the economic value of life due to the increased number of deaths at discharge). An increase of more than 1 µg·m-3 in ambient levels of PM2.5 was especially common during the study period: such events occurred on more than 122 days each year within each ZIP Code of residence considered.

This study brings researchers and clinicians one step closer to understanding the relationship between short-term exposure to PM2.5 and human health problems. The newly identified PM2.5-related diseases suggest that the effect of PM2.5 exposure is not restricted within individual organs. Short-term PM2.5 exposure has a more systemic effect on multiple pathophysiological processes such as inflammation, infection, and water electrolyte balance. The fact that hospitalization risks occurred when daily PM2.5 levels were below the current WHO air quality guideline suggests a need for stricter guideline values. Further, the health care costs associated with these health problems suggests large benefits in reducing short-term PM2.5 by even small amounts.

Pure air, noted Hippocrates, is fundamental to human health. However, progress in reducing PM2.5 has stalled in North America and Europe recently, and air pollution levels in low- and middle-income countries have risen alarmingly. To ensure that all populations have access to pure air and good health, we must fully define the effect of air pollution on human health, and then work to reduce it.

Yaguang Wei, doctoral student, Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston

Francesca Dominici, Clarence James Gamble professor of biostatistics, population, and data science, Department of Biostatistics, Harvard T.H. Chan School of Public Health, Boston.

Joel D Schwartz, professor, Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, MA, USA

Competing interests: None declared.