Marijuana grow operation (MGO or "grow op") is a place where marijuana is — or was — grown illegally. MGOs can range in size from one to thousands of marijuana plants and can occupy one room or an entire building. Indoor marijuana grow operations may contribute to poor air quality caused by marijuana mould growth and/or lingering chemical residues in the environment it was found. MGOs may also contribute to adverse health effects. Marijuana mould spores can be released into the air when a moldy plant is damaged or disturbed. You could be exposed to marijuana mould if you breathe in mould spores, breathe in the toxins that certain moulds produce, swallow or inhale moldy marijuana.

With marijuana mould exposure, some people may be more sensitive than others to fertilizer and pesticide residues. Being exposed to pesticides does not automatically mean that individuals will suffer a negative health effect. The toxicity of the specific pesticide, the concentration, how long an individual was exposed and the frequency of exposure, may contribute to an adverse reaction to chemical residues.