Indoor Humidity and Health

The 30 to 50 Percent Window

Below 30% relative humidity, mucous membranes dry out, skin cracks, respiratory irritation increases, and viruses survive longer on surfaces. Above 50%, mold can grow on surfaces and in concealed spaces, dust mites reproduce faster, and condensation forms on cooler surfaces. The sweet spot is 30 to 50%, and most homes swing outside this range seasonally: too dry in winter when heating systems run, too humid in summer or in homes with moisture intrusion from basements, crawlspaces, or inadequate exhaust ventilation.

Humidity Is a Root Cause, Not Just a Number

Many indoor air quality symptoms, from congestion to mold odors to skin irritation, trace back to humidity being out of range. The assessment evaluates humidity conditions alongside the factors that drive them: HVAC behavior, ventilation, moisture sources, and building characteristics.