Rating Concentrations of Mold Spores

What are Acceptable Concentrations of Mold Spores?

Mold testing results sometimes include the total concentration of mold spores, living and dead, expressed as the number of spores per cubic meter of air. Although outdoor spore concentrations can vary from hundreds of thousands to millions, total concentrations indoors usually vary from fewer than 100 to over 10,000 spores per cubic meter of air.

Rating Concentrations of Mold Spores  - High, Medium or Low

According to the American Academy of Allergy, Asthma, and Immunology (MAAI), fewer than 6,500 spores per cubic meter of outside air is considered a “low” concentration, and above 13,000 is a “high” concentration. Mold inspection showing concentrations between low and high are considered “moderate,” and a concentration above 50,000 is considered “very high.” At the “very high” level nearly all sensitized individuals will experience symptoms outdoors, whereas only the highly sensitized will experience symptoms at the “low” level. The MAAI also says that at the middle level of “moderate,” about 10,000 spores per cubic meter, many sensitized individuals will have symptoms.

Since there are 1,000 litres of air in a cubic meter, the middle level of “moderate” represents a concentration of 10 spores per litre of air, or about 5 per breath (assuming about 0.5 litre per breath).

Rating concentrations of mold sporesEffects of Mold Spore Concentrations Depend on Sensitivity

Just taking a breath of mold contaminated air can cause coughing, even in a space with few particulates of any type but containing fewer than 500 spores per cubic meter of air. At this concentration there is, on average, 1 spore per 2 litres of air, and therefore there are likely to be no spores in a fraction of a breath. Other very sensitized individuals describe similar immediate responses upon entering contaminated spaces. This suggests the likelihood of particulates smaller than spores that may be serving as surrogate carriers of mold allergens.

If you have culturable mold testing done in your home and the spore concentration is well in excess of 300 cfu/m3, you are in that 7 percent, and you may have a mold problem. (Of course, as always, indoor concentrations should be compared with those outdoors, which are usually higher.) An individual fungal species should not contribute more than 50 cfu/m3 to the total spore concentration, with the exception of Cladosporium spores (the most common outdoor fungus and thus frequently found indoors).

Rating Concentrations of Mold Spores - Weather Variables

In outdoor air, the species and spore concentrations vary according to the season and weather conditions. On a windy day the type and concentrations of spores can even change from moment to moment, as moldy leaves are blown around and spores are carried on airflows. During the spring, summer, and fall there may be thousands of spores per cubic meter of air, whereas in the winter there may be few if any.

Mold species and spore concentrations can vary indoors too, because spores enter buildings through open windows and doors or on people’s clothing or shoes, but if molds are growing in carpeting or in the heating or cooling system, the spores or allergens from the spores will almost always be present. Although these spore concentrations may vary drastically, depending on whether the mold has been disturbed, as long as indoor conditions remain the same, the varieties of mold that grow indoors will not change that much.

Because we spend so much time in buildings, our exposures to the limited types of spores from molds growing indoors can be fairly constant. People may be more likely to develop an allergy to indoor mold in a contaminated building than to develop an allergy to outdoor mold. And the more sensitive to mold the people in your household are, the more worried you should be about even low levels of spores indoors. And, of course, the presence of a potentially toxic species in an air sample taken in a room that is highly trafficked is reason for heightened concern.

Keep in mind though, if no one in your family is sensitive or allergic to mold and the reported concentrations are low, there is less reason for concern.