Most mulch is safe for humans, but a few types carry real risks

Standard wood mulch made from bark, wood chips, or shredded hardwood is not toxic to humans. You can touch it, work with it, and walk on it without poisoning. The danger comes from what mulch is made from — specifically, mulch treated with chemical preservatives, mulch made from contaminated wood, or mulch that harbors mold or fungi that produce toxins.

The mulch sold at garden centers for landscaping is usually safe. The mulch you need to worry about is mulch made from recycled pressure-treated lumber, mulch from demolition sites, or mulch that has been sitting wet long enough to grow certain molds. If you know what went into your mulch, you can decide whether it belongs near where you or your children spend time.

Key Takeaways

  • Untreated wood mulch from bark or hardwood chips poses no toxicity risk to humans through skin contact or incidental ingestion.
  • Mulch made from pressure-treated lumber contains arsenic and copper compounds that can leach into soil and pose a risk, especially for children who put hands in their mouths.
  • Mulch stored in wet conditions can grow molds that produce aflatoxins or other mycotoxins, which are harmful if inhaled or ingested in quantity.
  • Cocoa mulch, made from cocoa bean shells, contains theobromine and is toxic to dogs but poses no direct toxicity risk to humans at typical exposure levels.

Pressure-treated mulch and the arsenic concern

Pressure-treated wood was commonly made with chromated copper arsenate (CCA) until 2004, when manufacturers stopped using it for residential products in the United States. That wood is still in the ground in many yards, and some recycled mulch is made from it. CCA-treated wood contains arsenic, which can leach into soil, especially in wet conditions.

The risk is highest for young children who play in mulch, put their hands in their mouths, and do this repeatedly over months or years. Arsenic accumulates in the body. A single afternoon in treated-wood mulch will not poison a child, but regular play in it can raise arsenic levels in blood and urine. If you do not know the source of your mulch, assume it may be from treated wood and keep children's play areas away from it, or replace it with mulch you know is untreated.

Mulch labeled as "recycled wood" or "ground pallets" is a common source of treated-wood mulch, because pallets and construction waste often contain CCA-treated lumber. Ask the supplier whether the mulch is made from virgin wood or recycled sources, and whether it has been tested for arsenic.

Mold and mycotoxins in damp mulch

Mulch that stays wet develops mold. Some molds produce mycotoxins — poisons made by the fungus itself. The most common concern is aflatoxin, produced by Aspergillus species. Aflatoxin is a known carcinogen and can damage the liver. It becomes a health risk when inhaled in large amounts or ingested regularly.

The risk is real but not common in typical yard mulch. It is highest in mulch that has been stored in piles in warm, humid conditions for weeks or months without drying out. Mulch in a sunny, well-drained yard that dries between rains is unlikely to harbor dangerous mold levels. If you notice a strong musty smell or visible mold growth on mulch, it is a sign that conditions have been wet enough for mold to thrive.

People with mold sensitivity or respiratory conditions like asthma should be cautious around damp mulch piles. Turning the pile to increase air flow and letting it dry will reduce mold growth. If you are buying mulch in bulk, inspect it for smell and visible mold before delivery.

Cocoa mulch and theobromine

Cocoa mulch is made from the shells left over after cocoa beans are processed. It smells like chocolate and is popular in ornamental gardens. It contains theobromine, the same alkaloid found in chocolate, which is toxic to dogs at high doses but poses no direct toxicity risk to humans at the exposure levels that come from handling or living near cocoa mulch.

A human would have to ingest a large amount of cocoa mulch to experience theobromine poisoning — far more than would happen by accident. The real concern with cocoa mulch is that dogs will eat it because it smells like chocolate, and dogs are much more sensitive to theobromine than humans are. If you have dogs, avoid cocoa mulch entirely. If you do not, it is safe for human use.

Skin contact and minor ingestion

Touching untreated wood mulch does not poison you. Wood fibers do not absorb through skin, and the compounds in bark and wood are not toxic at skin-contact levels. Children who play in mulch and get it on their hands and clothes are not at risk from the mulch itself.

Accidental ingestion of a small amount of mulch — a child eating a handful, or swallowing some while playing — will not cause poisoning from untreated mulch. The risk comes from repeated ingestion over time, or from mulch that is contaminated with arsenic or mold. If a child swallows mulch once, there is no need to panic. If a child is regularly eating mulch, that is a behavior worth addressing, and you should know what the mulch is made from.

How to choose safe mulch for your yard

Buy mulch from a supplier who can tell you the source. "Virgin wood mulch" or "hardwood mulch" made from tree trimmings and bark is the safest choice. Ask whether the mulch has been tested for arsenic or other contaminants. Many landscape suppliers can provide this information.

Avoid mulch labeled as "recycled wood," "ground pallets," or "demolition mulch" unless you have confirmation that it does not contain pressure-treated wood. If you are buying in bulk, ask to see a sample and inspect it for smell, visible mold, and color. Fresh mulch should smell like wood, not musty or sour.

Store mulch in a location where it can dry between rains. A pile that stays wet is more likely to develop mold. If you are using mulch in a children's play area, choose virgin hardwood mulch from a reputable supplier and replace it every two to three years, as it breaks down and can accumulate contaminants over time.

Frequently Asked Questions

Can mulch give you a fungal infection?

Mulch itself does not cause fungal infections. However, fungi that live in mulch can cause skin infections if you have an open wound and the fungus enters through it. Wash any cuts or scrapes after working with mulch, and keep them covered if you are handling mulch regularly. The risk is low with normal yard work.

Is it safe to use mulch around a vegetable garden?

Untreated wood mulch is safe around vegetables. Do not use mulch made from pressure-treated wood, as arsenic can leach into soil and be taken up by plants. Virgin hardwood or bark mulch is the right choice for food gardens.

What should I do if I think my mulch contains treated wood?

Replace it with mulch from a known source. If you cannot replace it when ready, keep children away from it and wash their hands after any contact. If you are concerned about arsenic in your soil, you can have the soil tested through your local extension office.

Does mulch color matter for safety?

Dyed mulch is safe. The dyes used are typically iron oxide or other non-toxic colorants. Red, brown, and black mulch are all safe choices. The color does not indicate whether the mulch is treated or untreated — that depends on the source material.