Seasonal allergy is an immune system response to airborne substances like pollen or mold spores that appear during specific times of the year. This condition affects millions annually, causing symptoms such as sneezing, runny nose, and itchy eyes. Allergy season duration is not fixed and varies significantly.
Understanding Typical Allergy Season Duration
Allergy season spans several months, from early spring to late autumn. This extended period occurs because different plants release pollen at different times. For example, tree pollen is prevalent in spring, while grass pollen dominates late spring and summer. Weed pollen, such as ragweed, emerges in late summer and fall.
The staggered release of allergens means individuals sensitive to multiple pollen types may experience symptoms across several seasons. Wind-pollinated plants, which produce small, light, dry pollen grains, are the primary culprits. These microscopic grains travel great distances, easily entering the eyes, nose, and lungs, triggering allergic reactions.
What Makes Allergy Season Longer or Shorter?
Environmental factors influence the length and intensity of allergy season. Temperature plays a role, as warmer air and higher temperatures increase pollen counts. A mild winter can lead to an earlier start, with plants pollinating sooner. Conversely, a sudden temperature drop or freeze can halt pollen production.
Precipitation also affects pollen levels. Heavy rainfall can temporarily wash pollen out of the air, reducing counts, but a wet growing season can promote robust plant growth, leading to more pollen later. Dry, windy conditions are problematic for allergy sufferers, as wind spreads pollen and other airborne allergens over wider areas.
How Allergy Season Varies by Region and Time of Year
The timing and specific allergens of allergy season differ across geographic regions. In the Northeast, tree pollen season starts in March, followed by grass pollen from late May to July, and ragweed in late August and September. Southern states, with milder winters, experience tree pollen as early as late January or February, and ragweed season may extend from August through November.
The Midwest mirrors the Northeast, with tree pollen in early spring, grass pollen in late spring and summer, and ragweed from August through October. In the Western United States, especially coastal areas, tree pollen appears from January to June, with grass pollen becoming an issue in late spring and summer. Mountainous regions have shorter pollen seasons, while arid areas may have unique allergens like sagebrush.
Why Allergy Seasons Are Changing
Allergy seasons are becoming longer and more intense in recent decades, a trend linked to climate shifts. Warmer temperatures contribute to more frost-free days, allowing plants to bloom earlier and continue pollen production for extended periods. For instance, the freeze-free growing season has lengthened by an average of 19 days in many U.S. cities since 1970.
Increased atmospheric carbon dioxide levels also contribute, as plants use CO2 for growth, leading to greater pollen production. This combination of higher temperatures and increased CO2 leads to more abundant and potent pollen, exacerbating allergy symptoms. A 2021 study observed that pollen concentrations in North America increased by over 21% from 1990 to 2018, with the pollen season lasting more than 20 days longer.