The health impacts of air pollution are both cumulative and irreversible. The more polluted the air, the more severe the effects. The more polluted air you breathe over time, the more severe the effects. That is to say breathing "Moderate" quality air for 3 days might be similar to breathing "Unhealthy for Sensitive Group" quality air for 1 day. In both cases, your respiratory, circulatory, neurological, and immune systems have been damaged.
You can (and should) check your local air quality several times a day at AirNow.gov. The air quality index (AQI) goes from 0 to 500. An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.
EPA establishes an AQI for five major air pollutants regulated by the Clean Air Act. Each of these pollutants has a national air quality standard set by EPA to protect public health:
Following is an explanation of two of these pollutants -- particle pollution and ground-level ozone.
Particle pollution is similar to smoking cigarettes. The more you inhale, the worse it is for you. Also, the more particles in the air you breath, the worse it is for you. Breathing air on a day with an AQI that is unhealthy is similar to smoking a half-pack of cigarettes. If you go running instead of walking, it might be equivalent to smoking 2 packs of cigarettes.
The more particulate matter you breathe in over time, the greater the cumulative effect. The impact on your health will depend on your predisposition. In all cases your risk increases for nasal and upper respiratory tract health problems, heart attacks, strokes, asthma, and bronchitis, as well as premature death from heart ailments, lung disease, and cancer. Studies show that exposure can impair brain development in children and are significantly associated with the development of dementia and Alzheimer's disease.
Some of the impacts of particle pollution may be reversed overtime if further exposure to particle pollution is eliminated. After five years of quitting smoking, the risk of developing complications decreases by up to 50%.
The best lifestyle is to avoid particulate matter. Exercise outside only on days with a "Good" AQI under 50. If you must go out on days with an AQI over 50, wear an N95 mask. Indoors build and run a Corsi-Rosenthal Box filter.
"Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides, which are emitted by automobiles, gasoline vapors, fossil fuel power plants, refineries, and certain other industries." -- National Center for Atmospheric Research
Ozone primarily affects the respiratory and immune systems. The damage is permanent, untreatable and often results in death. The Santa Barbara County Air Pollution Control District reports, "Roughly one out of three people in the U.S. is at risk of experiencing ozone-related health effects."
The best lifestyle is to avoid ozone exposure. Since heat and ultraviolet light are needed to create ozone, staying indoors offers the best protection. Neither masks nor air purifiers protect from ozone exposure. (Ozone is O3 and almost the same size as Oxygen O2)
REFERENCES & RESOURCES
Indoor Air Pollution: How to Purify the Air, Aromatic Plants, and Phytoncides
Corsi-Rosenthal Box filter
Death by Ozone
COVID-19 and Air Pollution
Ozone Know Zone
Air Pollution Kills