The prevalence of asthma in the United States (U.S.) has doubled since 1970, coinciding with the increased use of gypsum-drywall in home construction. Mold growth is promoted when gypsum-drywall gets wet. Since asthma is linked to mold exposures, accurate quantification of mold contamination in homes is critical. Therefore, qPCR assays were created and then used to quantify 36 common molds in dust collected in representative U.S. homes during the first American Health Homes Survey (AHHS). The concentrations of the 36 molds, i.e. 26 water-damage molds (Group 1) and 10 outside molds (Group 2), were used in the formulation of a home's Environmental Relative Moldiness Index (ERMI) value. The ERMI values for each of the AHHS homes were assembled from lowest to highest to create the ERMI scale, which ranges from -10 to 20. Subsequent epidemiological studies consistently demonstrated that higher ERMI values were linked to asthma development, reduced lung capacity or occupant asthma. Reducing mold exposures by remediation or with HEPA filtration resulted in a reduced prevalence of asthma and improvements in respiratory health. The ERMI scale has also been successfully applied in evaluating mold concentrations in schools and large buildings and appears to have applications outside the U.S.