It says in another place on the website:
Silica. Crystalline silica is a known human carcinogen (IARC 2004). Silica in cosmetics may be in the form of tiny particles of glass (Merck 2004), sand or bits of ground quartz from mining operations (NIOSH 2002), or even an extract from horsetail plants (see, for example,
www.wholehealthmd.com, 2004). Of these, the sand or quartz material is the crystalline form known to be carcinogenic.
Other forms of silica — glass, for instance — are not considered known human carcinogens. In sales materials, the chemical giant Merck describes the company's technology breakthrough in producing of “tiny glass beads” that are now used in namebrand toothpastes and sunscreens (Merck 2004), and a company called SCHOTT promotes the use of a material called bioactive glass powder for cosmetic applications (SCHOTT 2004). The State of New Jersey cites “limited evidence that fibrous glass implanted into animals” causes cancer (NJDHHS 2004), but the glass form of silica is not classified as a human carcinogen.
A consumer has no way to know if the “silica” listed on the ingredient label of a product is a respirable, carcinogenic, crystalline silica or another form of silica that may not pose cancer risks through inhalation. Through ingredient label searches, EWG identified some form of silica in one of every nine products (862 products altogether), including 14 products in powder or aerosol form for which inhalation is likely. Regardless of the cancer risk, consumers may want to avoid the use of powders, even those that do not contain silica, because of potential risks for other kinds of lung damage. In consumer education materials, FDA notes that “powders may cause lung damage if inhaled regularly” (FDA 2000a).
So I think that horsetail is fine, it's just the crystalline forms that pose a problem. Most of the products that we use on a day to day basis contain numerous problematic ingredients. Check your skin lotions, they probably contain parabens, which is suspected of being carcinogenic as well.