Aspergillus is a saprophytic fungus that helps remove environmental carbon and nitrogen from the earth’s atmosphere. Aspergillus is most commonly found in soil, but its spores propagate rapidly in the air with each fungus capable of producing thousands of conidia.

These spores are commonly spread through environmental disturbances and strong air currents, that allow them to be found both indoors and out. Aspergillus spores are tiny, even by biological standards, allowing them to travel great distances in the air.

Why is Aspergillus dangerous to cannabis users?


Aspergillus is especially dangerous to immunocompromised individuals, in whom the airborne spores can lead to a debilitating invasive infection called aspergillosis, which oftentimes proves to be fatal.

A 1992 study found the incidence of cases of aspergillosis per year to be 1 – 2 patients per 100,000 people; however, this number is likely much higher today due to the increased use of immunosuppressants and stem cell therapy.

A 1983 study isolated Aspergillus fumigatus spores from cannabis smoke, indicating the spores do in fact survive combustion. There are a variety of factors that can increase an individual’s likelihood of developing aspergillosis including, but not limited to:

  • The ingestion of immunosuppressive drugs (after undergoing bone or organ transplants)


  • Low white blood cell count (resulting from chemotherapy) 


  • Asthma or cystic fibrosis 


  • Long-term corticosteroid therapy

While immunocompromised individuals are at greater risk of Aspergillus infection, there has been at least one confirmed case of aspergillosis developing in an immunocompetent cannabis user. There are several hundred different species of Aspergillus, and not all are dangerous to humans. A. fumigatus is the species that most commonly causes infection, and it has been linked to the death of at least one immunocompromised cannabis user. A. flavus, A. niger, and A. terreus have also been shown to cause infection.  

Aspergillus grows poorly on various culture-based microbial tests. As a result, there are no Aspergillus-specific culture tests on the market. This is likely due to Aspergillus forming heterogeneous macro-colonies in solution and thus single CFUs are actually derived from a clump of hundreds to thousands of viable cells.

Only DNA-based testing can accurately differentiate between Aspergillus species. This is important because only specific species cause Aspergillosis, the potentially deadly lung infection.

Labs that rely on visual inspection to differentiate Aspergillus species may incorrectly identify A. brasiliensis as A. niger, which will then lead to false failures.