Wang B, Eum KD, Kazemiparkouhi F, et al. The impact of long-term PM2.5 exposure on specific causes of death: exposure-response curves and effect modification among 53 million U.S. Medicare beneficiaries. Environ Health. 2020;19(1):20.
Observational. Researchers used hybrid machine learning and Cox proportional hazards models to assess the association of long-term exposure to PM2.5 (particulate matter 2.5 microns or less in width) with specific causes of death.
Fifty-three million (53,000,000) U.S. Medicare beneficiaries (aged ≥65 years) from 2000 to 2008 with approximately 4 billion person-months of follow-up.
Researchers calculated exposure to PM2.5 through a combination of Environmental Protection Agency (EPA), meteorological, and geo-temporal models applied to each participant’s address and zip code.
Disease-specific and all-cause mortality.
PM2.5 was associated with increased mortality from cardiovascular disease (CVD), respiratory disease, and cancer (with the exception of lung cancer). A 10 μg/m3 increase in average PM2.5 in the 12 months prior to death was associated with a 5% increase in all-cause mortality, as well as an 8.8% increase in all CVD-related mortality, a 5.6% increase in all respiratory-related mortality, and 2.5% increase in all cancer-related mortality in models adjusted for age, gender, race, ZIP code, and socio-economic status (SES). PM2.5 exposures, however, were not associated with lung cancer mortality. Results were not sensitive to control for ozone exposures. PM2.5-mortality associations for CVD- and respiratory-related causes were positive and significant for beneficiaries irrespective of their sex, race, age, SES, and urbanicity, with no evidence of a lower threshold for response or of lower risk ratios (RRs) at low PM2.5 levels. Associations between PM2.5 and CVD- and respiratory-related mortality were linear and were higher for younger, Black, and urban beneficiaries, but were largely similar by SES.
This study, along with the recent meta-analysis by Pranata et al,1 was particularly relevant while we were in a lockdown response to the Covid-19 pandemic.2 The prescribed social-distancing response to the pandemic had a striking side effect: Global air pollution levels dropped dramatically across the world.
The result is that we have become participants in a global experiment, a global cohort testing whether lowered air pollution is associated with improved health, decreased morbidity, and a reduction in mortality, particularly from CVD. Air pollution levels fell dramatically as each country instituted lockdown measures to slow the spread of Covid-19.3
Pranata et al’s paper was published on March 13, 2020, in the Journal of Evidence Based Medicine. It is a comprehensive review and meta-analysis of past studies on air pollution and CVD mortality. Data from 84 cohorts, comprising a total of 28,215,394 subjects, were combined. Increases in fine particulate air pollutants were associated with increases in all measures of CVD, acute coronary events, stroke, and high blood pressure. Mortality rates from CVD increased by 10% for the PM2.5 increases and 17% when the larger PM10 particles were tracked. Increases in nitric oxide (NO) are associated with a 17% increase in CVD mortality and a 23% increase in all-cause mortality.3 Either of these 2 recent papers are suitable candidates for review.
Air pollution levels fell dramatically as each country instituted lockdown measures to slow the spread of Covid-19.
A paper by Hayes et al from July 2019 also deserves mention as it reported that each 10 μg/m3 increase in PM2.5 was associated (in fully adjusted models) with a 16% increase in mortality from ischemic heart disease (hazard ratio [HR] 1.16; 95% CI 1.09-1.22) and a 14% increase in mortality from stroke (HR: 1.14; 95% CI: 1.02-1.27).4 Any of these papers would be sufficient to predict that the decrease in air pollution secondary to the Covid-19 response might impact current disease rates.
Wang et al’s finding that lung cancer mortality was not associated with air pollution was not unexpected. Prior studies have also reported a similar null effect,5 though others have seen a positive association.6
The range of predictive numbers from worsening air quality that we find in this quick review varies from a 5% increase in all-cause mortality (Wang et al), to a 10% increase in CVD mortality (Pranata et al), to a 16% increase in mortality from ischemic heart disease (Hayes et al). Though a few years older, the 2015 study by Pope et al reported that a similar increase in fine particulate levels was associated with a 12% increase in CVD deaths.7
These findings provide a gauge to estimate what we may see in future statistics that look back on this time period in regard to CVD mortality.
In an article posted in early March on the academic website G-Feed, Marshall Burke, a professor at Stanford’s Earth System Science Department, calculated that the decreased air pollution in China this past winter may have saved 20 times more lives in China than were lost due to the infection in that country.8 Burke relied on older Chinese research to make his calculation; neither Pranata’s nor Wang’s results had been published at the time he made his calculations.9
Burke utilized the 2016 findings from Su et al and data collected during the 2008 Summer Olympics and Paralympic Games. Recall how China went to great efforts to reduce ambient air pollution during the Games by restricting traffic.10 Burke estimated that this resulted in “about a 10 ug/m3 reduction in PM across China in Jan-Feb of 2020 relative to the same months in the previous 2 years.” Guojun He et al had reported “that a 10% decrease in concentrations reduces the monthly standardized all-cause mortality rate by 8%.”11
Burke writes, “Putting these numbers together…yields some very large reductions in premature mortality...I calculate that having 2 months of 10 ug/m3 reductions in PM2.5 likely has saved the lives of 4,000 kids under 5 and 73,000 adults over 70 in China.”
The U.S. started out with much cleaner air than China, so perhaps these relationships will not apply. Yet, as already mentioned, Wang et al found “no evidence of a lower threshold for response or of lower risk ratios (RRs) at low PM2.5 levels.” This suggests that lowering our own pollution levels may still result in significant improvements.
About 647,000 people die in the U.S. each year from CVD.12 If our staying at home were to lower this figure by 10% (a conservative estimate), that would prevent nearly 65,000 deaths from CVD alone, a change that would be noticeable. Pranata et al’s 16% decrease would mean over 103,000 lives saved.
In past issues of NMJ we have reviewed numerous studies that have associated PM2.5 exposure with various measures of health and disease. In the coming months and years, we may see more accurate measures of air pollution’s true impact and perhaps be able to more accurately calculate its costs to communal health.
Unfortunately, our experiment may be confounded by other concurrent events. The EPA is currently rolling back pollution enforcement rules, and it may be difficult to account for the harm these rollbacks may cause when balanced against the health improvements from cleaner air. Job loss and the related shifts that unemployment have on heart health outcomes will also need to be taken into account.
As we hunkered down during stay-at-home orders, a second unplanned experiment occurred related to climate change. Carbon emissions have dropped dramatically. An abrupt halt in Chinese industrial manufacturing apparently decreased their carbon emissions by 25% earlier this year.13 Whether this effect will last long enough to measure or be buried beneath the impact of economic stimulus packages that favor use of fossil fuels is yet to be seen.
Time will tell, but in the meantime, those of us who live in urban areas got to enjoy cleaner air and clearer skies, even if it was only from our front porch and back patio.