Arsenic in the biochar, and what you can do

Compare a soil test, not the biochar test, with these figures. The U.S. EPA residential soil screening level for inorganic arsenic in the November 2024 tables is 0.68 mg/kg. That is a screening level, not a cleanup standard and not a prediction that someone will become ill. Oregon DEQ’s published residential direct-contact risk-based concentration for arsenic is 0.43 mg/kg. Oregon Health Authority’s yard-and-garden guide lists Portland Basin background arsenic at 8.8 mg/kg. Other Oregon regions are often about 7 to 19 mg/kg.

A recommended 5% to 10% mix does not put 70 ppm into the soil. The biochar’s share of that mix is about 3.5 to 7 ppm, before the arsenic that was already in the ground. Whether the finished bed is above a screening level depends on that soil test, not on the biochar label.

Inorganic arsenic is toxic, and the dose depends on the soil people actually touch, not on 70 ppm in the unused biochar. At the recommended mix, that soil concentration is much lower. The exposure that matters is usually swallowing small amounts of soil or dust over time, especially for young children, plus eating food grown in the amended soil. Washing produce removes dirt on the surface. It does not remove arsenic taken up inside the plant.

Long-term ingestion of inorganic arsenic is associated with skin changes and with higher risk of skin, bladder, and lung cancer, among other effects. Touching soil once is not the exposure that causes sudden poisoning. This page cannot calculate anyone’s dose. The customer letter still asks you not to eat crops grown with this biochar while the investigation continues.

If someone has swallowed soil and is ill, or you are worried about a child who eats dirt from the bed, contact a physician or Poison Control at 1-800-222-1222.

The letter asks you not to remove biochar that is already mixed in, and not to eat crops grown with it. Those two requests can both be met if new food is grown in clean soil that does not mix with the affected media.

Use a lined raised bed or containers filled with clean soil or commercial potting mix. Keep a barrier between that clean soil and the biochar-amended soil so roots and watering do not pull from the old media. Do not dump, give away, or landfill the old soil to clear the space. Email [email protected] before you move it.

Ornamental plants in the existing soil avoid the food pathway. They do not remove the arsenic. Do not treat that planting as a cleanup.

Wash hands after contact with the soil, and have children wash hands and faces before they eat. Take shoes off at the door. Damp-mop or use a HEPA vacuum instead of dry-sweeping dust. Cover bare spots with grass, mulch, or landscape fabric plus a clean cover so people and pets are not playing in loose soil. Wash outdoor toys and pets. Do not eat, drink, or smoke while working in that soil. Do not compost the affected media into a new food bed. These steps limit contact. They are not a substitute for the requests in the customer letter.

A test of the bed, not the biochar bag, is the soil number. At a 5% to 10% mix that number should be much lower than 70 ppm. Ask an accredited lab for total arsenic in soil and say the sample is for a garden or yard. If you used more than 5% to 10%, email us before you rely on a guess. If you drink from a well and the material was used near it, a separate arsenic test of the well water is a reasonable precaution. A soil number does not measure the well. Do not dig the biochar out in order to sample it.

Product questions go to [email protected]. Health questions go to your clinician or your local public health department. The customer letter is on the official statement page. CO2T will share more when the investigation produces results.