Weather conditions can determine not only how much mold is in the air, but which types of mold spores are circulating.
Some molds release spores during warm, dry, and windy afternoons, while others thrive in damp conditions and peak before dawn.
Rain can initially send more spores into the air, but prolonged or heavy rain may eventually wash them out, providing allergy sufferers with some relief.
If your allergy symptoms seem to get worse at certain times of day — or suddenly improve after a rainstorm — the weather may have something to do with it.
The Weather Company reports that mold spores generally fall into two groups based on the conditions that cause them to become airborne. The distinction may help explain why someone with a mold allergy feels fine one day but develops sneezing, watery eyes or breathing problems the next.
One group of mold thrives under warm, dry, and breezy conditions. These spores are capable of surviving relatively harsh conditions and can be picked up and carried through the air by even a light breeze.
Concentrations of these dry-weather spores tend to peak during the afternoon, when humidity is lower and winds may be stronger. Their numbers generally decline when humidity reaches its highest point, often just before sunrise.
Some molds prefer moisture
A second group behaves almost exactly the opposite way.
These damp-weather molds depend on moisture to release their spores. As moisture condenses around a spore, changes in surface tension can eventually propel it into the air.
As a result, damp-weather mold counts tend to peak before dawn, when humidity is highest. Morning dew can also help send these spores into the air.
That means the time of day could matter for people with mold allergies. Depending on the specific mold that triggers their symptoms, some people may experience greater exposure during a dry afternoon while others could be more affected during humid early-morning hours.
Rain can make things better — or worse
Rain has a more complicated effect on mold allergies.
The first drops of rain striking leaves and other surfaces can shake loose mold spores, temporarily increasing the amount circulating in the air. Moisture can also encourage the release of spores from damp-weather molds.
But if the rain continues, the effect can reverse. Rain can capture airborne spores and carry them to the ground. Heavy rain may therefore reduce airborne mold concentrations and ease symptoms such as watery eyes and breathing difficulties.
Mold counts don't tell the whole story
For consumers with allergies, simply knowing that the mold count is high may not provide all the information they need.
Different molds have different allergy risk thresholds. The Weather Company notes that a relatively uncommon mold may trigger allergic reactions at concentrations far below those of a much more abundant mold.
In one example, allergists set the risk threshold for a less common mold about 10 times lower than for a more prevalent variety. In other words, the mold with the highest spore count isn't necessarily the one making you miserable.
Mold allergies can cause coughing, sneezing, itchy skin, and breathing problems, and people with asthma may experience more serious symptoms. Mold commonly thrives in damp environments, including bathrooms and basements, as well as outdoors.
For allergy sufferers, paying attention to humidity, wind, rainfall, and the time of day — rather than simply looking at an overall mold count — may provide additional clues about when symptoms are most likely to flare.