How Small is a Mold Spore? Mold spores are extremely tiny — most range from 1 to 30 microns in size. For perspective, a single human hair is about 70 microns thick. That means mold spores are often invisible to the naked eye and can easily float through the air, reaching deep inside your walls, carpets, HVAC systems, and lungs. Because of their microscopic size, professional mold testing is essential to detect and identify elevated mold levels before they cause serious property damage or health