Microbial growth in ventilation systems may contaminate indoor air and cause a variety of problems.
Fungal growth is potentially the most problematic for air quality.
Some fungi can cause lung infections. Many fungi can cause allergic reactions in susceptible people and respiratory irritation in non-allergic people. Inhalation of fungal spores by highly susceptible people can have fatal consequences. Some environmental bacteria can grow in ventilation systems, but these are raely a threat to healthy people. They can, however, be a nosocomial problem.