Illinois Mold Season: The Moisture Calendar

Illinois does not have one season, it has two, and they run on different physics. The outdoor moisture load peaks in July, when the dew point averages 65.5°F, which the National Weather Service describes as "lots of moisture in the air, becoming oppressive". Months averaging in the Weather Service's oppressive band run July. The cold-season problem is different: in January, averaging 27.7°F outdoors, the moisture that condenses on cold surfaces is generated inside the house, not brought in from outdoors. In Illinois the cold-season problem is the bigger of the two. These are outdoor climate figures, not measurements of any building.

By dew point, which is the measure of how much water the air actually holds, July leads at 65.5°F, with NOAA's 90th percentile for that month at 73.4°F. January is the driest at 20.4°F. Months at or above the Weather Service's "sticky" line: June through September. Note that relative humidity would give you a different and less useful ranking, because it rises every night as the air cools without any extra water being present.

Because the glass is the coldest surface in the room and the moisture is already indoors. In January, Illinois outdoor air averages a dew point of 20.4°F; warm that same air to 70F indoors and add nothing to it and it lands at roughly 15% relative humidity. Anything above that came from inside: showers, cooking, laundry, houseplants, people. EPA's own line on it, verbatim: "If you see condensation or moisture collecting on windows, walls or pipes act quickly to dry the wet surface and reduce the moisture/water source. Condensation can be a sign of high humidity." EPA's prevention lists, reproduced in full on this page, cover what to do about it.

EPA has answered this directly: "Molds come in many colors including white. "Black mold" is not a species or specific kind of mold, and neither is "toxic mold." Sometimes the news media use the terms "toxic mold" and "black mold" to refer to molds that may produce mycotoxins or for a specific mold, Stachybotrys chartarum. Molds that produce mycotoxins are often referred to as toxigenic fungi." EPA also writes: "No one can tell whether a mold is producing mycotoxins just by looking at it." Every statement about mold itself on this page is EPA's, quoted with a link.

EPA has written about this directly: "Is sampling for mold needed? In most cases, if visible mold growth is present, sampling is unnecessary. Since no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building’s compliance with federal mold standards." EPA also notes: "Please note that this document presents recommendations. EPA does not regulate mold or mold spores in indoor air" For anything beyond that, EPA's guide and your state or local health department are the sources.

EPA's own wording, quoted rather than summarised: "Who should do the cleanup depends on a number of factors. One consideration is the size of the mold problem. If the moldy area is less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself, follow the guidelines." And for the larger case: "If there has been a lot of water damage, and/or mold growth covers more than 10 square feet, consult EPA’s Mold Remediation in Schools and Commercial Buildings. Although focused on schools and commercial buildings, this document is applicable to other building types." EPA does not say to hire someone above ten square feet; it routes you to a second EPA document. EPA adds separately: "If you have health concerns, consult a health professional before starting cleanup."

This page is a climatology: it describes what a normal July in Illinois looks like over the 1991-2020 period, not what this particular year is doing, so the year to year answer is in the actual weather. It also measures the outdoors. Whether a given building gets wet depends on its drainage, its grading, its insulation, its ventilation and its leaks at least as much as on the weather around it. EPA puts the useful part in one sentence: "The key to mold control is moisture control."

Verbatim: "Keep indoor humidity low. If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity. Relative humidity can be measured with a moisture or humidity meter, a small, inexpensive ($10-$50) instrument available at many hardware stores." That figure is EPA's, it is national, and it does not change from state to state. What changes from state to state, and what this page is actually for, is how hard the outdoor climate makes it to hold. In July in Illinois the outdoor dew point averages 65.5°F; in January the same outdoor air warmed to 70F indoors would sit near 15%.

From NOAA NCEI's 1991-2020 U.S. Climate Normals, hourly product, which publishes an official normal dew point and temperature for every hour of a normal year. We average those hours into calendar months for each Illinois station that reports around the clock, then average the stations equally. Stations publishing only part of the day were rejected rather than included, because a daytime-only record would bias every monthly figure. The dew point bands are the National Weather Service's, quoted. The snapshot is rebuilt when NOAA publishes a new normals period.