Adding to the causal evidence, there’s a 2019 paper that uses wind direction as an instrumental variable for PM2.5. They find that IV > OLS, implying that observational studies are biased downwards:
Comparing the OLS estimates to the IV estimates in Tables 2 and 3 provides strong evidence that observational studies of the relationship between air pollution and health outcomes suffer from significant bias: virtually all our OLS estimates are smaller than the corresponding IV estimates. If the only source of bias were classical measurement error, which causes attenuation, we would not expect to see significantly negative OLS estimates. Thus, other biases, such as changes in economic activity that are correlated with both hospitalization patterns and pollution, appear to be a concern even when working with high-frequency data.
They also compare their results to the epidemiology literature:
To facilitate comparison to two studies from the epidemiological literature with settings similar to ours, we have also estimated the effect of PM 2.5 on one-day mortality and hospitalizations [...] Using data from 27 large US cities from 1997 to 2002, Franklin, Zeka, and Schwartz (2007) reports that a 10 μg/m3 increase in daily PM 2.5 exposure increases all-cause mortality for those aged 75 and above by 1.66 percent. Our one-day IV estimate for 75+ year-olds [...] is an increase of 2.97 percent [...]
On the hospitalization side, Dominici et al. (2006) uses Medicare claims data from US urban counties from 1999 to 2002 and finds an increase in elderly hospitalization rates associated with a 10 μg/m3 increase in daily PM 2.5 exposure ranging from 0.44 percent (for ischemic heart disease hospitalizations) to 1.28 percent (for heart failure hospitalizations). We estimate that a 10 μg/m3 increase in daily PM 2.5 increases one-day all-cause hospitalizations by 2.22 percent [...], which is 70 percent larger than the heart failure estimate and over five times larger than the ischemic heart disease estimate. Overall, these comparisons suggest that observational studies may systematically underestimate the health effects of acute pollution exposure.
Adding to the causal evidence, there’s a 2019 paper that uses wind direction as an instrumental variable for PM2.5. They find that IV > OLS, implying that observational studies are biased downwards:
They also compare their results to the epidemiology literature: