Houston Pollen & Mold — FAQ

Every count on this site comes from the Houston Health Department, which operates a pollen and mold monitoring station for the City of Houston. We publish their measured numbers — we do not generate our own estimates.

It is an actual measured count. Many popular weather and allergy sites show a forecast — a computer model's guess at tomorrow's pollen based on weather and plant models. Houston's count is the real amount of pollen captured from the air and identified under a microscope. Measured data tells you what is actually in the air, not what an algorithm predicts.

A spore trap pulls a steady volume of outdoor air across a sticky surface for 24 hours. A trained analyst then examines the sample under a microscope, identifies each pollen grain and mold spore by type, and counts them. The result is expressed as grains (or spores) per cubic meter of air.

They group the raw count into severity bands. The cutoffs are different for each pollen type and for mold, because a number that is high for grass is low for mold. The exact ranges are listed in the table above. In general, most people with allergies start to notice symptoms at Medium and feel them clearly at Heavy or above.

Any day graded Heavy or Extremely heavy for a pollen type you are sensitive to. For trees that means roughly 90 grains/m³ or more; for grass about 200 or more; for weeds about 500 or more. Mold uses a much larger scale and reaches Heavy in the tens of thousands of spores per cubic meter.

Tree pollen runs from late January through May, peaking in February through April, with oak as the main driver. Grass pollinates over a long warm season from roughly March into October. Weed pollen — led by ragweed — peaks in the fall, from August through November. Mold can stay elevated nearly year-round because of Houston's heat and humidity.

Ragweed season in Houston typically runs from mid-August through November, with the worst weeks in September and October before the first strong cold front. Ragweed is the leading cause of fall hay fever and its pollen can travel for miles.

The Health Department collects and reads samples on weekdays only, and does not report on weekends or City of Houston holidays. When you see no new report, it usually means the lab was closed that day, not that pollen was zero.

No. Pollen comes from trees, grasses and weeds; mold spores come from fungi. They are counted with the same air-sampling method and can cause similar allergy symptoms, so Houston tracks them together, but mold is graded on its own separate scale. These are outdoor air counts and do not measure mold inside your home.

Common steps include keeping windows closed and using air conditioning, staying indoors in the morning when counts are often highest, showering and changing clothes after time outside, and checking the count before outdoor activities. For diagnosis and treatment, see a board-certified allergist — this site is informational and not medical advice.