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The prevalence of allergies among Americans has skyrocketed. In 1970, about one in ten Americans suffered from hay fever, which is caused by airborne allergens, such as pollen and mold spores; by 2000, three in ten did. Overall, allergies cost the United States more than $18 billion per year. Asthma — which often occurs alongside pollen allergies — has become more common, too. Today, some 6.2 million Americans under the age of 18 suffer from the chronic disease.


Data showing comprehensive, historical pollen trends on the local level are hard to access in the United States. But analysis of temperature data from 201 cities shows that the growing season, or the period between the last and first freezes of the year, has lengthened by an average of two weeks since the 1970s, as the planet has warmed. A longer growing season often means pollen allergies earlier in the spring, and later into the fall.

By the end of the century, unchecked emissions would add an additional month or more to the growing season in most of the country.


CO2 emissions warm the atmosphere and extend the growing season. But more CO2 also directly encourages some plants to produce more pollen. At atmospheric CO2 concentrations of 600 parts per million (ppm) — which we could reach by 2060 if we maintain current rates of emissions — ragweed pollen production could doublerelative to 2000 CO2 levels, laboratory experiments have shown. We are already well above 2000 CO2 levels, having increased concentrations from about 370 ppm to roughly 410 ppm since that year. That’s not all: at 600 ppm, each grain of ragweed pollen could get 1.7 times more allergenic, or allergically potent.


  • For data on the growing season from the 201 weather stations assessed, click here.
  • For Climate Central’s short report on climate change and pollen, click here.
  • For our partner story with the Rivard Report, click here.

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