Air quality is a matter of particulates in the air, dew point, barometric pressure, smoke, motor vehicle emissions, factory pollutions, weather inversions, etc. Air quality of 18 is really good, so there's a whole lot less in the air to make breathing hard; that's why you're breathing better. Rain can reduce the smoke, emissions, pollutions by washing them out of the atmosphere, but in areas where there's factory pollution, lots of smoke or emissions, it can sometimes make breathing worse because of the humidity. Anything under 50 is good; 50 to 100 is generally moderate, and over 100 most of us with COPD have some trouble and those with severe or very severe COPD have a LOT of trouble.

Appreciate your answer. I have difficulty at 30 or more. We had a rain all night. No factory near. But, a lot of gravel dust. I'm in the country. So that explains alot to me. Thank you so much.

Smog, dirty air is measured as air quality. Higher the number, poorer the air quality

what are you referring to as air quality? Humidity. barometric pressure or what?

Thanks , RickMartin2, it rained all night. So it must of cleared the air.