Question: What is the “normal / acceptable” level of total aerobic bacteria and yeast & mold in homes / buildings
Answer: There are no universally agreed acceptable levels of total aerobic bacteria, yeast and mold in buildings. However, you may find levels suggested by various organisations. I would say that since there will always be bacteria/mold/yeast in houses, any levels that appear abnormally high is uncceptable. However, apart from the levels, you have also to consider what organisms are present since some are more harmful or destructive than others.
The tables below show some levels published by the Commission of the European Communities in Indoor Air Quality & its Impact on Man: Report No. 12: Biological Particles in Indoor Environments. ECSC-EEC-EAEC, Brussels-Luxembourg, 1993.
Categories of CFU/m3 (mixed populations of fungi) obtained with the Andersen six-stage sampler in combination with MEA, and with the N6-Andersen one-stage sampler in combination with MEA and DG18.
| Category | Fungal Counts (CFU/m3) in Houses | Fungal Counts (CFU/m3) in non-industrial indoor environments |
|---|---|---|
| Very low | <50 | <25 |
| Low | <200 | <1,000 |
| Intermediate | <1,000 | <500 |
| High | <10,000 | <2,000 |
| Very High | >10,000 | >2,000 |
N.B.: These categories are based on the range of values obtained in indoor environments and not on a health risk evaluation.
Categories of CFU/g dust (mixed populations of fungi) obtained by direct
plating on V8 or DG18, and by suspension in peptone followed by plating on V8 or DG18, for houses and other non-industrial indoor environments
| Category | Method | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| V8/directa | DG18/directa | V8/peptoneb | DG18/peptoneb | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Very low | <1,000 | <2,000 | <10,000 | <10,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Low | <2,500 | <6,000 | <20,000 | <20,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Intermediate | <5,000 | <9,000 | <40,000 | <50,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| High | <10,000 | <15,000 | <100,000 | <120,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Very High | >10,000 | >15,000 | >100,000 | >120,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
a : direct plating of 30 mg dust
b: 100 mg dust suspended in a pepton solution (1 : 50), 0.1 ml of the solution plated on the medium
N.B. : These categories are based on the range of values obtained in indoor environments and not on a health risk evaluation.
Categories of CFU/m3 (mixed populations of bacteria) obtained with the
Andersen six-stage sampler or slit sampler (sampling time 10 – 15 min.,
incubation at 20 – 25°C for 3 – 5 days), for houses and non-industrial indoor environments.
| Category | Bacterial Counts (CFU/m3) in Houses | Bacterial Counts (CFU/m3) in non-industrial indoor environments |
|---|---|---|
| Very Low | <100 | <50 |
| Low | <500 | <100 |
| Intermediate | <2,500 | <500 |
| High | <10,000 | <2,000 |
| Very High | >10,000 | >2,000 |
N.B.: These categories are based on the range of values obtained in indoor environments and not on a health risk evaluation