How It's Measured: Counting centers use microscopes to count grains of pollen, says Mike Seidel, a meteorologist for The Weather Channel. Then the pollen-count level is categorized (green, yellow, orange, or red) for each type of pollen (mold, grass, tree, or weed).
When You Feel It: People with allergies will feel symptoms when the count tips into the yellow range.
What to Do: Because pollen counts are highest from 2 a.m. to 10 a.m., don't sleep with your windows open, advises Dr. Pamela A. Georgeson, president of Kenwood Allergy and Asthma Center in Chesterfield Township, Michigan. Run the air conditioner to filter allergens. Before bed, take a shower to remove pollen that may be sticking to you.
Where to Look: Refer to the American Academy of Allergy, Asthma and Immunology site at aaaai.org to find pollen-counting centers near you.
How It's Measured: AQI is a composite of five pollutants, including ground-level ozone. It's keyed to a number from 0 to 500 and a color chart: green, yellow, orange, and red.
When You Feel It: It AQI is usually higher in the late afternoon and early evening in the warm months of the year. At the yellow and orange levels, people with respiratory ailments are more likely to feel symptoms. "Some people have described the symptoms like getting a sunburn inside your lungs," says Dr. Norman H. Edelman, of the American Lung Association.
What to Do: Some doctors may suggest taking an extra dose of medication on high days, but you should check with yours to see if that's a good idea.
Where to Look: Go to airnow.gov.
How It's Measured: The UV outlook tells how much sunburn-causing radiation is coming from the sun to your shoulders. Based on a scale of 0 to 10+, the UV outlook is ranked low (0-2), moderate (3-5), high (6-7), very high (8-10), and extreme (10+).
When You Feel It: When the UV outlook reaches the high levels of 6 to 7, alerts go out. However, if you have fair skin, your potential for burn begins even in the moderate range, notes Dr. Mary P. Lupo, a dermatologist in New Orleans.
What to Do: "The most effective protection against UV is with products with mexoryl," Lupo says.
Where to Look: The Weather Channel offers hourly UV outlooks. Check it out at weather.com.
How It's Measured: The heat index measures how hot it feels based on air temperature and moisture in the air. It has four warning levels: caution, extreme caution, danger, and extreme danger.
When You Feel It: The heat index is usually highest between 4 and 5 p.m.
What to Do: Drink fluids, even if you don't feel thirsty. If you plan on being outside doing strenuous activity, do it when the index is lowest, usually early in the morning around sunrise.
Where to Look: The National Weather Service charts the heat index calculator from their Web site at nws.noaa.gov.