Half Moon Bay Mold Forecast
Mold Today
Today (20 September) mold in Half Moon Bay is low; alternaria and cladosporium are low.
Mold Tomorrow
Tomorrow (21 September 2026) levels are similar to today.
7-Day Trend
Over the next week alternaria and cladosporium will increase.
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Mold Risk
Today - 20 September
| Mold species | Level | Value |
|---|---|---|
| Alternaria | Low | 10/100 |
| Cladosporium | Low | 18/100 |
Tomorrow - 21 September
| Mold species | Level | Value | 7-Day Trend |
|---|---|---|---|
| Alternaria | Low | 13/100 | Steady |
| Cladosporium | Low | 20/100 | Steady |
Local Mold Conditions
With 39.1% average soil moisture, Half Moon Bay sits in a temperate climate — one where warmth and moisture need to coincide for mold to grow, and those peaks don't always line up. The supply of organic matter here is moderate. Neither species is well-placed here — Cladosporium is limited by moisture, Alternaria by a combination of moisture and temperature.
In Half Moon Bay's temperate climate, the mold season falls where the wet and warm periods overlap — not simply at the wettest point. Moisture reaches 61.3% in January and drops to its lowest in August. Where that wet period aligns with the warm season is when mold is most active. Airborne spore counts for both species peak as the ground dries — as dry-spored fungi, they release spores in dry, windy conditions, not rain.
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Frequently Asked Questions About Mold in California
Why do I still have allergy symptoms after the pollen season?
Allergy symptoms after the main pollen season may be caused by mold spores. Outdoor mold levels can remain high into late summer and autumn, when pollen levels are already falling. Mold allergy can cause symptoms similar to pollen allergy, including sneezing, a runny or blocked nose, itchy eyes, coughing, and wheezing.
What are mold spores and why do they cause allergies?
Mold spores are tiny particles released by fungi into the air. When inhaled, they can trigger an allergic reaction in sensitive people. Common symptoms include sneezing, a runny or blocked nose, itchy or watery eyes, coughing, and wheezing.
When are mold spore levels highest?
Outdoor mold spore levels often increase during warm and humid conditions, particularly when there is plenty of decaying vegetation. Levels can vary considerably with weather, season, vegetation, and location, and some types of mold are more common during particular times of the year, e.g. in the Fall.
How can I reduce my exposure to mold spores?
If you are sensitive to mold, check local mold spore levels and forecasts before spending long periods outdoors. On high-risk days, reducing time around dense vegetation, compost, fallen leaves, and other sources of decaying plant material may help limit exposure.
Understanding the Scale
The 0–100 scores represent risk levels, not direct measurements of pollen or air pollutant concentrations. Higher scores indicate higher risk. Pollen and air quality use different thresholds.
Mold
| None | 0 | No or negligible risk of exposure. Pollen/mold exposure is expected to be minimal. |
| Low | 0-25 | Low risk of exposure. Small amounts of pollen or mold spores may be present. |
| Moderate | 25-50 | Moderate risk of exposure. Pollen/mold exposure is expected to be noticeable. |
| High | 50-75 | High risk of exposure. Elevated pollen/mold exposure is expected. |
| Very High | 75-100 | Very high risk of exposure. Pollen/mold exposure is expected to be at or near its highest levels. |
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