The U.S. Environmental Protection Agency (U.S. EPA) and many Public Health agencies have published what they consider to be "safe levels" of certain metals in soil based on their own research or that of other organizations. Unfortunately, these organizations do not always agree on what is safe, and "safe levels" specifically for gardens where food is grown can be higher than what is considered safe by other agencies.
The table below shows the most relevant "safe levels" for garden soil. The table includes:
As you can see, what is "safe" for home vegetables gardens is not easy to determine. There is not one value that each agency agrees upon. For example, for seven of the metals listed in the table, soil with
concentrations that are considered hazardous waste in California is below the upper limit that the U.S. EPA considers to
be safe for children under six to play in. Similarly, levels of lead, arsenic and cadmium considered
safe by the U.S. EPA for compost used in vegetable gardens are higher than
levels the California Office of Environmental Health Hazard Assessment
considers safe for human health.
Not included in the table, however, are the levels of each metal that occur naturally in the soil. While each location will have a different natural level, due to t he geologic history of the area, normal ranges of certain metals for average soils are: As (5-10 ppm)Cd (0.01-2.0 ppm), Cr (5-1500 ppm), Cu (2-250 ppm), Pb (2-300 ppm), and Ni (2-750 ppm) (Gardea-Torresdey, et al., 2005; Gadepalle, et al., 2008).
When the agencies were researching what they consider "safe" levels, they all considered the risk to children from directly eating soil, however only the U.S. EPA's Land Application of Biosolids considers human health risk due to eating garden produce. Children, however, are at greatest risk from exposure to metals in soils, with some agencies - including San Francisco’s Department of Public Health - stating that there is no safe level of lead in our body. The amount of metals we are exposed to from eating produce is far less than that of eating soil directly.
Since there is no simple answer as to what is considered “safe,” you can use the above table to help you decide what levels you feel are safe for your family. Then, by educating yourself and adopting a few common-sense actions, you can continue to eat fresh, healthy produce from your home vegetable garden. Further information about each of the "safe levels" can be found at the following websites: