1) All biological agents, including microbial pathogens, are classified into 4 categories according to their pathogenecity and epidemic danger, which is rated into BSL-1 (Biological Safety Level) to BSL-4 level, BSL-1 being the least pathogenic, BSL4 becoming the most dangerous one. Molds are rated into BSL-1 to BSL-3 level.  BSL-1 molds tend to be allergenic; BSL-2 molds pose dangers to immunocompromised people, including babies, elderly people, and patients under medical conditions; BSL-3 molds will infect healthy people. Environment with BSL-3 molds, or other biological, microbial agents, is in any way regarded as unsuitable for human to reside in. Molds Coccidioides immitis, Blastomyces dermatitidis and Histoplasma capsulatum, etc,  among a few others, are classified as a BSL-3 mold, which attacks healthy people. 


2)  Even though CDC (US Center for Diseases Control and Prevention) and EPA (US Environmental Protection Agency) don't have any laws or regulations to stipulate that certain amount of molds are deemed as dangerous. By convention, to immunocompetent (normal healthy) people, several thousand counts of mold spores in a 1000 L of air and above is regarded as unsuitable to live in a residence; But for immunocompromised people, including infants, elderly, and medical patients, several hundred counts of mold spores to thousands in a 1000 L of residence air are also considered dangerous to live in. To severely immunocompromised population, several hundred mold spores in a 1000L of air are deemed as dangerous.


3) According to clinical studies, Aspergillus fumigatus constitutes 90% of clinical cases of human mycosis, which is classified as a BSL-2 mold.  All other molds and yeasts constitute the remaining 10% of human cases with fungal infections. Other less commonly isolated molds and yeasts from humans are other species of Aspergillus, yeast Cryptococcus neoformans and yeast Candida albicans, Zygomycetes such as Mucor and Rhizopus (both of which are also black bread molds), and plant pathogen Fusarium et al.


4)   In indoor environment, empirically considered normal and relatively clean air should have mold counts lower than 200- 300 per 1000 L of air. Mold spores counts significantly above 500 to thousands are considered having slight to significant mold issue. Indoor air will remain clean only if the door and windows remain closed and the ventilation is on to filter air particulates and mold spores, the AC remains on when temperature is high and the humidity is high outside.


5) For molds to grow, there must have high humidity, high temperature and biodegradable materials (construction materials and spilled food, drinks). The air ventilation in a house should always on to filter air particulates and mold spores, good brands of air filters are necessary, such as using Filtrete brands. Cheap filter will not filter mold spores at all. The A/C system should always on to squeeze water molecules out of the indoor environment, reducing moisture for molds to grow when in summer time when the humidity and temperature are high. For Bacteria to grow, there must have excessive water on top of these conditions.