Hourly measurements of particulate matter that is smaller than 2.5 μm in diameter (PM2.5) have been made at air-monitoring sites in Hamilton and Burlington, Ontario, Canada, since 2003. These sites are separated by ~6 km; Burlington is right on Lake Ontario while Hamilton has, directly to the east, very heavy industry between it and Lake Ontario. Hence, by taking the difference between measurements at Hamilton and Burlington, it is possible to isolate, during east-wind conditions, PM2.5 that result from emissions from the industrial sectors (primarily steel mills) located in Hamilton’s northeast end. After screening the data for east winds off Lake Ontario, it was found that median background values of PM2.5, of 5–10 μg m−3 are increased by an additional 5–10 μg m−3 by emissions from local sources. On the contrary, however, industrial contributions to PM2.5 in Burlington during south winds are much smaller at ~3 μg m−3 (industrial sectors are due south of Burlington). This difference is likely due either to wind direction–dependent local circulation patterns or to alignment of sources that can concentrate PM2.5 into Hamilton. It was also found that throughout much of 2009, but especially during spring and early summer, the industrial contribution of PM2.5 at Hamilton was reduced relative to other years by amounts that are statistically significant at the 95% confidence level, even when measurements are augmented with large amounts of Gaussian noise. These reductions are consistent with documented reductions in steel production during the global economic crisis that peaked in the first half of 2009.