Chicago's allergy season is getting longer

Chicago's allergy season increased by 14 days, on average, from 1970 to 2021, per an analysis from Climate Central, a nonprofit climate news organization.

• That's based on the number of days between the last freeze each spring and the first freeze each fall — essentially, the annual window during which seasonal allergy sufferers are most likely to rely on their antihistamine of choice to get by each day.

The big picture: The average increase in allergy season days was 15 across about 200 cities Climate Central looked for that same period.

Why it matters: The lengthening allergy season is tied to climate change, per Climate Central, with big health ramifications for the roughly one-quarter of Americans who suffer from seasonal allergies — and for respiratory health more broadly.

• Earlier spring and longer periods of freeze-free days mean that plants have more time to flower and release allergy-inducing pollen.

What's more: From 1990 to 2018, pollen counts increased by 21% nationwide, with the greatest increases in the Midwest and Texas, according to a 2021 study, Axios' Arielle Dreher reports.

Zoom out: The "freeze-free" season has lengthened by at least a month in more than 30 cities, leaving places like Reno, Nevada, with a whopping 99 more allergy season days than it had in 1970.

Yes, but: Allergy seasons shortened in a handful of cities from 1970 to 2021, including Denver (-15 days) and Charlotte (-9 days).

Of note: Cities in the Deep South and parts of California were left out of the analysis, because they don't experience traditional freezing seasons the way other parts of the country do.

What's next: Ongoing climate change will drive further deviation from former norms.

• "We do expect that areas that haven't previously had substantial pollen seasons will potentially start to experience pollen seasons," William Anderegg, director of the Wilkes Center for Climate Science & Policy at the University of Utah, told Axios.