Butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are synthetic compounds that are commonly used as a preservative and antioxidant in pharmaceutical preparations and cosmetic formulations that contain oils and fats. Dermal exposure to BHA occurs from its use as an antioxidant in commercial products, especially lipstick and eye shadow.  BHT is a toluene-base ingredient that is used as a preservative in both food and personal care products.
While they are effective at preventing the oxidation of fats and oils, they have been found to have harmful effects on human health.

One of the main concerns with BHA and BHT is their potential to cause cancer. Studies in laboratory animals have shown that these compounds can cause tumors, particularly in the liver and stomach. While human studies have not definitively linked BHA and BHT to cancer, the International Agency for Research on Cancer (IARC) has classified BHA as a possible human carcinogen.
BHA and BHT may also have negative environmental effects.

These compounds are not biodegradable, which means that they can accumulate in the environment and potentially harm wildlife. This is a persistent bioaccumulative toxic ingredient. I have a rule if an ingredient alters the DNA, health or metabolic function of an animal than I do not want it in my products. My concern is low doses of the thousands of chemicals people are exposed to every day of their lives adds up and  many of them bioaccumulate. It's not  a good thing for young bodies that are developing to accumulate toxins. They enter into their adult years with a compromised immune system. As for adults, they accumulate chemicals their entire lives and suddenly metabolic functions go awry.

In addition to their potential carcinogenic effects, BHA and BHT have also been linked to other health problems. For example,  a study carried out in normal mammalian kidney cells found  exposure to BHA caused specific damage at the cellular level and exerted a significant cytotoxic effect even at low doses. A safety assessment of BHT reported that BHT applied to the skin of rats was associated with toxic effects in lung tissue, but judged that the low concentrations used in cosmetics were safe.  

So scientists conclude that a little bit of this toxicity doesn't affect negatively because it's is absorbed slowly and in low concentrations; therefore, they conclude it's not harmful. Well let's say that 's true, what happens when it bioaccumulates?

There is also a developmental and reproductive toxicity associated with these compounds.  Studies carried out in rats found that exposure to high doses of BHA resulted in weak dysfunction and underdevelopment of the reproductive systems of both male and female rats. Changes in testosterone levels, sex weight organs and sexual maturation were also observed. In addition, The American Conference of Governmental Industrial Hygienists (ACGIH) has determined that there is moderate evidence that BHT is a human respiratory irritant.

The California EPA’s Proposition 65 list identifies BHA as a possible human carcinogen and requires labeling for products that are used on the lips. Interestingly enough the International Agency for Research on Cancer (IARC) determined that there was only limited evidence of carcinogenicity for products used on the lips. The Environment Canada Domestic Substance List has classified BHT as expected to be toxic or harmful and classified BHA as a high human health priority.

Overall, while BHA and BHT may be effective preservatives, their potential health risks make them a cause for concern. On our prohibited ingredient list at the  Skin Curator we identified  BHT or BHA as ingredients we won't allow in our collection of products for the reasons set forth here.

Environmental Working Group, “Skin Deep. Butylated Hydroxyanisole,” [Online]. Available: http://www.ewg.org/skindeep/ingredient/700740/BHA/. Accessed April 18, 2022.

Labrador V et al., “Cytotoxicity of butylated hydroxyanisole in Vero cells,” Cell biology and toxicology, vol. 23, no. 3, pp. 189-99, 2007.

Lanigan RS, Yamarik TA, “Final report on the safety of assessment of BHT (1),” International journal of toxicology, vol. 21, no. Suppl 2, pp. 19-94, 2002.