100% Pure Horsetail Base Coat Polish

Reported use concentrations run from 0.002%-100% and any number over 50% usually comes from a nail product.

Actual skincare numbers run much lower: cleansers (0.3-26%), toners (0.07-7%), face moisturizers (0.04-0.2%).

The Cosmetic Ingredient Review Expert Panel reviewed it and concluded it is safe in the present practices of use and the International Agency for Research on Cancer classifies it as a non-carcinogenic to humans.

True allergy to this ingredient is uncommon but real: 44 out of 1,450 patients patch tested between 1992-2011 reacted with most cases clustering in people with leg ulcers, occupational hand eczema, or repeated use of Isopropyl Alcohol on already-broken skin.

Irritation and dryness are far more likely than allergy; before you throw away your skincare, just know this is dose-dependent and the risk is modest.

The irritant potential of the alcohols used in hand antiseptics is very low and skin irritation is significantly lower after alcohol application than after detergents (though isopropanol is somewhat more irritating than ethanol).

A 70% solution was a severe eye irritant in rabbit eye tests so the industry keeps this ingredient at low percentages in eye-area products.

One genuine caution: a 70% solution was a severe eye irritant in rabbit eye tests, so it belongs at low percentages in eye-area products, which is where the industry already keeps it.

According to the National Rosacea Society based in the US, you should be mindful of products with these alcohols in the top half of ingredients.

Another study using a cream with horsetail and soybean extract found 80% of participants showed at least a 26% improvement in forehead wrinkles after 8 weeks.

Animal research has shown topical Equisetum Arvense stimulated skin and tissue growth in mice.

The research so far is encouraging, but still in its early days. But it's a lovely supporting ingredient and can be a welcome addition to any routine.

It also naturally has a sweet, soft-rose scent.

The reason you'll almost never see it on a modern ingredient list comes down to safety data:

In a two-year dietary studies by the US National Toxicology program, benzophenone increased kidney and liver tumors in rats and mice.

This led the International Agency for Research on Cancer to classify it in Group 2B, "possibly carcinogenic to humans," and California to add it to the Proposition 65 list, effective June 22, 2012.

The EU took this a step further as well: after a harmonized classification as a category 1B carcinogen, Benzophenone was added to the list of substances prohibited in cosmetics, with the ban applicable from December 2023.

In practical terms, this is now a phased-out ingredient across major markets.

Its only main relevance today is as a trace impurity and breakdown product; octocrylene generates benzophenone through a retro-aldol reaction so it's treated as an impurity and degradation product that should be kept at trace levels.

On concentrations, neither the EU nor the FDA sets a hard number. The EU allows it with purity limits on leftover manufacturing residues and the FDA allows it in externally applied cosmetics at levels consistent with batch-certified good manufacturing practice.

Industry reports around 0.5% concentration usage in leave-on products, but most products use far less because it's a pretty strong dye.

The one rule that both regions share is that it isn't allowed anywhere it would touch mucous membranes (so no lipstick or eye products).

That restriction comes from an unresolved 2005 EU data request on how much of the dye penetrates skin and breaks down there, and not from any reported consumer harm.

Allergy risk is low and a guinea pig study ranked it a much weaker sensitizer than common hair-dye chemicals. But the same study found animals sensitized to paraphenylenediamine sometimes cross-reacted to it, so it's worth flagging for a patch test if you have a known permanant-hair-dye allergy.