Many supplements and supplemented foods have recently become subject to concern for their heavy metal content, especially in respect to the stringent requirements for labelling in the state of California. However, contrary to popular belief most supplements are relatively low in heavy metals when compared to common foods. Low levels of various heavy metals are common in nature and are present in a large number of the natural foods we eat day-to-day. Supplements, when made from quality ingredients, with good provenance and manufacturing standards are generally consistent with these normal levels of heavy metals found in food.

Of course, we don’t want to ever have higher than normal intakes of potentially dangerous heavy metals and minerals, but most recent discussion has centred on warnings on supplement or food labels related to Proposition 65, a California State legislation which specifies maximum allowable amounts of various chemicals in food, that are known causes of cancer, birth defects or other reproductive harms[1].

Proposition 65 (officially known as the Safe Drinking Water and Toxic Enforcement Act of 1986) came into force in California in 1986. The act intended to safeguard the safety of the State’s drinking water from contamination with chemicals known to cause cancer, birth defects, or other reproductive harm and requires businesses to inform Californians about exposures in their products to these chemicals. Certain levels of listed chemicals trigger that a warning must be applied to a product. If a business, product, or service cause exposure to listed chemicals greater than this amount (known as a ‘safe harbour level’), the business must provide a Proposition 65 warning. If there are no ‘safe harbour levels’ listed, the business that exposes individuals to that chemical are also required to provide a Proposition 65 warning.  

Are the levels in Four Sigmatic any concern?

Heavy metals including lead and others, are found naturally in many foods including fruits and vegetables, grains,  and nuts and seeds. They are also found in lesser amounts in meat, seafood, and dairy products. Because heavy metals are naturally occurring in soil, inevitably small amounts will be absorbed into many of the foods that we eat every day (Figure 1).

Figure 1. Lead content of selected foods per serve.

Four Sigmatic ensure and guarantee that each batch of products always contains less than 1 ppm of lead. (One part per million is equal to one drop of water diluted into 50 litres). 

So, why is there a warning on my product?

All Four Sigmatic labels are compliant with the strict Proposition 65 regulations in California, so, even if you live in NZ, there may be a warning label, but this is actually unnecessary in Aotearoa because the quality of the product exceeds all known quality standards. Four Sigmatic also ensure that all products are tested in third-party laboratories for heavy metals, along with microbial quality, aflatoxins, mycotoxins, mould, yeast, and pesticides.
Read more about Four Sigmatic’s testing here:


The levels of exposure to various chemicals as defined by Proposition 65 are far more restrictive than those suggested by other health organisations, including those of the World Health Organization. In fact, the levels of heavy metals in products like Four Sigmatic mushroom blends are consistent with or less than those found in many common fruits, vegetables, and tubers.

Four Sigmatic products are safe for consumption and offer a convenient, delicious, and healthy way to get some extra mushroom benefits into your life!



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