They differ from plants or animals in how they reproduce and grow. Allergic reactions to mold are most common from July to late summer. Although there are many types of molds, only a few dozen cause allergic reactions. Some common spores can be identified when viewed under a microscope. Some form recognizable growth patterns, or colonies.
Many molds grow on rotting logs and fallen leaves, in compost piles and on grasses and grains.
Unlike pollens, molds do not die with the first killing frost. Most outdoor molds become dormant during the winter. In the spring they grow on vegetation killed by the cold. Mold counts are likely to change quickly, depending on the weather. Certain spore types reach peak levels in dry, breezy weather.