Weed pollen

What it is

Weed pollen on a 0–5 scale, by kind of weed. Ragweed is the big one.

How it affects breathing

• Evidence. Mostly a hay-fever story; the asthma evidence is thin, so these warn later than grass does.
• How likely. Ragweed runs from mid-August to the first frost and is the most common pollen allergy in the eastern US, so nose and eye symptoms are near-certain for anyone sensitized. Asthma flares are less common and tend to come when a September cold lands on top of it.
• What helps. A daily antihistamine or steroid nose spray started before the season, a daily controller inhaler kept up through it, windows shut and a shower after time outdoors on high days. Allergy shots are the one treatment that changes the allergy itself rather than the symptoms.

How Breathing Index measures it

Your Breathing Index takes the day’s value, since Google’s model resolves pollen by the day and the weed-pollen evidence does not support a longer window.

Where the number comes from

Estimated by Google’s pollen model from land cover and weather, not counted with an instrument. A season calendar fills in where the model is silent, and says so.