I have long been concerned about the government's warnings about the fine-particle air pollution called PM2.5, which is hazardous because the particles are readily inhaled into human lungs, where they can be deposited, and some cause cancer.
The government has a color-coded way of telling us about the pollution in each region.
The colors range from "good" green (up to 12 micrograms of PM2.5 concentration per cubic meter), through yellow and orange to "unhealthy" red (starting at 55 micrograms). This coding suggests that we should avoid aerobic exercises on the red days.
But, when added up, five days of good air may be just as threatening to our health as one day of unhealthy air, and a whole year of "green" air is just about as bad as a single month of "red" air.
Conclusion: We need to curb pollution every day of every year, not just on the red days in January and July.
Salt Lake City