Outdoor allergy, Scientific article

US pollen season start and regional variability

Pollen allergy is one of the most common forms of allergy in the US. It affects approximately 19.2 million adults as well as 5.2 million children. Typically, symptoms of hay fever include nasal congestion and obstruction, sneezing fits, conjunctivitis, ear infections as well as allergic asthma.   

It is a regionally variable disease and heavily influenced by several environmental factors. It is most importantly defined the local flora, weather, climate, and severity of air pollution. 

For pollen allergy sufferers, knowledge about the beginning, intensity, and end of the main pollen season is important. 

Pollen season in the US 

The start dates for tree, grass, and weed pollen season depends strongly on latitude. General rule of thumb: the lower the latitude, the earlier the start date. The start date also correlates with the duration of pollen season. Areas with an earlier start date also have longer seasons.

Start date of pollen season

Knowledge about the pollen season start date is crucial for allergy sufferers, as antihistamine and anti-inflammatory allergy medication can take up to 4 weeks to take full effect.  

We divide allergy symptoms into three categories by severity: low (approx. 10-20 pollen grains/m3, moderate (approx. 50-90 grains/m3), and severe (approx. 80-90 grains/m3). The start of the pollen season is defined as the day when the pollen concentration reaches a threshold of 50 pollen grain*day/m3. Very sensitive allergy patients experience symptoms even before this threshold is reached.  

End date of pollen season

The end date of pollen season is defined similarly to the start: the date at which the integral pollen concentration in the air is less than 50 pollen grain*day/m3. 

Region-specific pollen calendars

To illustrate how the start of the pollen season varies in different areas with different climates, the graphic below shows the season start and pollen concentration for various allergens in four different regions: Seattle, Washington; San Jose, California; Waco, Texas; and London, Ontario.  

Seattle, Washington

Located in the Pacific Northwest, the climate in Seattle is cool and moist, with dry summers and wet, snowless winters. The region is dominated by evergreen trees, which is illustrated by the abundance of Cupressaceae pollen (cypress family) in the airborne pollen composition at over 37 percent. Since the Cupressaceae pollen dominate in this area, the start of the pollen season in Seattle is primarily governed by the bloom of Cupressaceae trees. Grass and weed pollen only constitute very little of the overall pollen constitution at 2.9 percent and 1.3 percent, respectively.  

Seattle’s pollen season is relatively short since it only detects very little pollen after the month of July.  

San Jose, California

San Jose¬†is in central California with a warm, mild Mediterranean climate, with most of its rainfall happening in the winter. Due to their presence throughout their year, San Jose is one of the few areas where researchers sample pollen all year round. Tree pollen are most dominant once again at 94 percent of all identified and sampled pollen, but compared to Seattle, there is not one sort of tree pollen dominating ‚ÄstUlmus¬†(elm¬†family)¬†and¬†Olea¬†(olive family)¬†pollen¬†are most abundant at 14 percent each.¬†Morus¬†(commonly known as mulberries)¬†and¬†Pinaceae¬†(pine family)¬†pollen are the next most dominant at¬†around 12 percent each.¬†Of the four most dominant pollen,¬†Olea¬†is the most potent,¬†Pinaceae¬†is the¬†mildest.¬†¬†


Diagrams of pollen calendars for Seattle, WA and San Jose, CA


London, Ontario

Surrounded by the great lakes, London, Ontario experiences cold winters and humid summers. The pollen constitution in London is more diverse. Still, tree pollen dominate the overall constitution at 81 percent, with weeds and grasses coming in at 13 and 6 percent, respectively.  

As illustrated in the graph, London’s main pollen season runs from March through September. Tree pollen season ends when grass pollen season begins around late May. Regarding weed pollen, Ambrosia (commonly known as ragweed) is the most dominant between mid-August and mid-September.  

Waco, Texas

The climate in Waco, Texas is humid and subtropical, constituting of hot dry summers with rains in mid to late spring. Like other stations in warmer climates, Waco has significant pollen concentration throughout the year, with the lowest concentration in July. This stands in stark contrast with more Northern stations, where the lowest pollen concentration occurs in the winter months. Waco experiences three peaks in pollen season: the earliest peak happens in January with Cupressaceae pollen, the main peak happens between late March and late April with Quercus (oak family) pollen being the most abundant at almost 20 percent, along with Carya (commonly known as hickory), Acer (commonly known as maples) and Ulmus pollen. The third and last peak occurs in mid-September through October, with Ambrosia and other weed pollen types dominating. 


Diagrams of pollen calendars for London, Ontario and Waco, Texas


Source: Figure 4 | Pollen calendars and maps of allergenic pollen in North America | SpringerLink 

How to use a pollen forecast calendar 

The National Allergy Bureau (NAB) publishes a pollen forecast calendar based on pollen counts reported by nationwide counting stations. Allergy levels in specific areas can be viewed here: click. 

For pollen allergy sufferers, the pollen forecast calendar is a useful toolkit for checking concentration and activity of certain allergens in the air. Since patterns in specific regions repeat themselves in a very similar fashion every year, consulting a pollen forecast calendar helps allergy sufferers to prepare for allergy seasons and make necessary precautions. 

During peak allergy season for their specific allergen, allergy sufferers should check a pollen forecast calendar on a day-to-day basis and medicate accordingly (in consultation with their GP or allergy specialist) before heading outside.  

Research has found that on an average day, pollen counts are usually highest in the morning, peak around midday and then gradually fall as the day continues. Usually, the pollen count is lowest in the late afternoon before dawn, or in the early evening. If possible, allergy sufferers with severe symptoms benefit from scheduling their daily outside activities accordingly.  

Sources: aaaai.org, ecowatch.com, alabamaallergy.com, link.springer.com



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