Pollen and Bioaerosols

What it is

Bioaerosols are airborne particles of biological origin. The category covers pollen grains, fungal (mold) spores, bacteria, viruses, dust mite fragments, cockroach allergens, and pet dander. Sizes range from about 0.02 micrometers for viruses to 100 micrometers for the largest pollens.

Pollen and mold spores are the two main outdoor bioaerosols that affect health. Indoors, dust mites, pet dander, and mold add a year-round allergen load.

Where it comes from

Tree pollen dominates in early spring (oak, birch, maple, cedar, juniper). Grass pollen peaks in late spring and early summer. Weed pollen, especially ragweed, runs from late summer through the first fall frost. A single ragweed plant can release a billion pollen grains per season.

Mold spores rise in warm, damp conditions: spring rains, late summer, and after rain or irrigation. Outdoor levels often dwarf indoor levels, but indoor mold colonies in damp basements, around leaky pipes, in HVAC systems, or behind walls can sustain high exposure year-round.

Climate change has lengthened the North American pollen season by roughly 20 days since 1990, and pollen production per plant has increased about 20% over the same period.

Health effects

Roughly 25% of US adults and 19% of US children have a seasonal allergy. Symptoms include sneezing, runny or stuffy nose, itchy eyes, postnasal drip, and fatigue (allergic rhinitis, often called hay fever).

Pollen and mold are major asthma triggers. Emergency room visits for asthma spike during high-pollen periods and after thunderstorms, which can rupture pollen grains into smaller, more inhalable fragments (so-called "thunderstorm asthma"). Long-term exposure to indoor mold is linked to new asthma cases in children, respiratory infections, and worsened existing asthma. Some molds produce mycotoxins; chronic exposure to heavy contamination has been associated with hypersensitivity pneumonitis.

How it's measured and typical levels

Pollen and spore counts are reported as grains per cubic meter of air, sampled with rotating-drum or volumetric samplers and counted under a microscope. The National Allergy Bureau (NAB) certifies stations across the US.

Typical pollen scale (grains/m³, tree or grass):

Low: below 15 (tree), below 5 (grass)
Moderate: 15 to 90 (tree), 5 to 20 (grass)
High: 90 to 1,500 (tree), 20 to 200 (grass)
Very high: above 1,500 (tree), above 200 (grass)

Mold spore counts above 13,000 grains/m³ are considered high. Indoor mold should be visually absent; routine air sampling is not usually recommended unless there is a known problem.

What you can do

Track local pollen forecasts (NAB-certified counts at pollen.aaaai.org). On high-pollen days, keep windows closed, run AC, shower and change clothes after being outside, and rinse pets that have been outdoors. Sunglasses and a hat reduce eye and hair pollen pickup.

Indoors, a HEPA air purifier in the bedroom cuts allergen exposure during sleep. Wash bedding weekly in hot water (130°F / 54°C). Allergen-proof mattress and pillow encasements help with dust mites. Keep indoor humidity at 30 to 50% to limit dust mite and mold growth.

For mold: fix the moisture source first; cleaning without addressing leaks or condensation just delays regrowth. Use a HEPA vacuum and N95 mask when cleaning visible mold under 10 square feet. Larger areas need a remediation professional. For persistent symptoms, ask a doctor about allergy testing and immunotherapy.

Sources

This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.