Is Moldy Compost Dangerous? (Here’s the Truth)

compost pile

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It is common for compost to get moldy when it is moist over a period of time. Is moldy compost dangerous?

Moldy or decaying compost can carry airborne toxic bacteria that can result in the deadly Legionnaire disease after inhaling.  A mask should be worn when handling compost because the mold spores can get stirred up by just opening a bag of compost, adding new materials, or turning the compost.

In this article, I will describe the dangers related to handling moldy compost.  If you have asthma or respiratory conditions, be sure to read towards the end to learn about the precautions you can take when composting.

How harmful are compost spores?

Inhaling the spores from decayed or moldy plant materials can cause a deadly lung infection according to medical professionals.

This has happened to a 47-year-old man in the United Kingdom, who died in intensive care one week after opening bags of rotting plant materials in the garden (The Guardian, June 2008).

Investigations found that he had a bacterial infection after inhaling spores from the Aspergillus fumigatus fungus which is found on dead and decaying leaves.  Initially, he felt chest pains and breathing difficulties but the problem later developed into pneumonia and damaged the lungs. 

Such a problem is not uncommon and has been documented various times with people coming into contact with mold (EI-Tawil & EI-Tawil, 2003).

The spores can cause harmless allergic reactions but can also cause serious reactions when too many spores get into the lungs.  Even just opening a bag of organic waste can cause mold spores to be stirred up and can damage the lungs.

Gardeners who handle organic waste should wear face masks.

What is the risk with moldy compost?

The main risks associated with moldy compost are inhaling and accidentally ingesting mold spores. Mold spores are incredibly tiny, so they can easily be inhaled like dust and pollen. Once they get into the lungs they can create an infection, and partially block the inside of your lungs.

For certain types of very toxic molds, if you inhale large amounts it can be a very serious medical condition regardless of how healthy you are.

1. Legionnaires’ disease

The Legionella bacteria (e.g. the Legionella longbeachae species) is toxic and is commonly found in compost and potting soils.

A study found that over two-thirds of the compost samples examined were colonized by the toxic Legionella bacteria (Conza, Pagani & Gaia, 2013).  Also, composting facilities are major sources of Legionella infections and the pathogen can be further redistributed when the compost or potting soil is used in gardens and agriculture.

Compost produces dust or bioaerosols which are tiny particles that can penetrate the lungs.  These tiny particles carry live or dead microorganisms, such as spores, fungi, bacteria or protozoa, allergens, and toxins. 

When breathed in, it can cause a mild flu-like illness known as Pontiac fever or a severe pneumonia-type illness called Legionnaires’ disease.

2. Weil’s disease

Compost that are frequented by rats can carry rodent urine and can cause Weil’s disease.

Compost is attractive to rats looking for food scraps and so they can frequent compost and leave urine in the pile.  The urine from rats often carries a disease called Weil’s disease.  It can be concentrated in a small area of the compost or spread throughout the compost with moisture.

The disease can cause mild symptoms such as headaches, and muscle pain, but can also in some cases cause severe reactions such as organ failure and internal bleeding.

Infection can happen easily when handling compost without washing your hands afterward.


If you have respiratory conditions, have asthma or easily get allergic reactions to dust such as sneezing, or sore throat, the best is to avoid composting. 

Alternatively, you can take precautions before you compost.  These are:

  • Don’t turn your compost on windy days.
  • Wear a face mask
  • Spray water over your compost pile to make it moist, not wet, so that the spores are not easily lifted when turning.
  • Wear impermeable gloves, such as nitrile gloves which are also waterproof
  • Turn the compost gently to not kick up mold spores as much
  • Wash your hand thoroughly directly after working with your compost
  • Do not keep compost in an enclosed area to create a concentration of mold spores around your compost heap. An open area with good ventilation will help carry away mold spores.

How to get rid of mold in your lungs?

According to medical experts, salt therapy or “halotherapy” is helpful for getting rid of mold in the lungs (source).  

Hydrotherapy is typically done at an alternative health clinic, involving breathing in the salty air. They will have you sit in a room that is filled with different types of salt.

If you live close to a beach, going for a long walk for 30 minutes or an hour each day can increase the salt content in your lungs. This will have a similar effect to a halotherapy treatment.

Do not go swimming in cold water if you have flu-like symptoms because the cold water can cause your flu to worsen.

Sweating such as receiving saunas has also been proven to help the body to detoxify (Mahlouji et al., 2020).

How easy is it to breathe in compost fumes?

It’s relatively easy to breathe in compost fumes.

It does not have to be in a windy environment. Even just opening a bag of moldy compost will release bioaerosols from the compost.


In general, moldy compost is dangerous and can be deadly in some cases, causing Legionnaire’s disease or Weil’s disease.  The airborne pathogens can coat your lungs easily when you open a bag of compost or potting soil. 

The best precaution is to wear a face mask, wear gloves and not turn it on a windy day. 

Apart from causing respiratory conditions, your compost pile can also self-ignite when conditions are met. Be sure to read the next article on self-combustion of compost piles.

Happy gardening!


7 Conditions When Your Compost Can (Actually) Self-Ignite


Adetunji, J. (2008, June 12). Man dies after inhaling fungal spores from garden compost.

Conza, L., Pagani, S. C. & Gaia, V. (2013). Presence of Legionella and Free-Living Amoebae in Composts and Bioaerosols from Composting Facilities. PLoS ONE 8(7): e68244.

Compost can produce dust or bioaerosols that are contaminated with harmful bacteria.

EI-Tawil, S. & EI-Tawil, T. (2003). Lord Carnarvon’s death: the curse of aspergillosis? The Lancet, vol. 362 (9386).

Gromicko, N. Compost Pile Hazards.

Mahlouji, M., Alizadeh, V. M., Dadmehr, M., Rezaeizadeh, H., Nazem, E. & Tajadini, H. (2020).  Sweating as a Preventive Care and Treatment Strategy in Traditional Persian Medicine.

Telloian, C. (2022, March 9). You Don’t Need to ‘Detox’ After Mold Exposure — Here’s What to Do Instead.

The Guardian (June, 2008).  Man dies after inhaling fungal spores from garden compost.

Wéry N. (2014). Bioaerosols from composting facilities–a review. Front Cell Infect Microbiol.

Carol Chung

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