Aluminium in antiperspirants
Antiperspirants contain ingredients called aluminium salts (sometimes referred to as aluminium/zirconium salts) that dissolve in sweat and leave a thin coating of gel over the sweat glands. This coating reduces the amount of sweat on the skin for a number of hours after the antiperspirant is applied. Alum, a salt of aluminium, is the crystal widely used in "natural" deodorants/antiperspirants and works along similar lines.
Aluminium is the third most naturally abundant element in the environment, found in food, water and pharmaceuticals as well as a wide range of consumer products. There is no safety data that suggests that aluminium presents a health threat when included in antiperspirants.
Although there is no evidence to prove it, some have questioned whether antiperspirants could be linked in some way to breast cancer. Why? The answer is that several studies have demonstrated a negligible potential for aluminium salts to penetrate into (but not through) the skin. However, if a small amount were absorbed from antiperspirant, this would be tiny in comparison to the amounts we consume in the foods we eat daily. After all, antiperspirants are designed to work by staying on the surface of the skin, so the products would not work if a significant amount of the active ingredient was absorbed into the skin itself.
Throughout 2019, an independent panel of top European scientific experts spent many months investigating the safety of aluminium in cosmetic products, which includes antiperspirants and deodorants. This expert panel is called the Scientific Committee on Consumer Safety (SCCS). In March 2020, the SCCS published a detailed opinion which reconfirms the fact that aluminium is safe in all cosmetic products, and does not cause cancer. The experts didn’t just look at cosmetics in isolation, they also took into account exposure from other sources such as diet, to make sure that the overall exposure is safe.
A number of leading cancer research organisations also support the view that there is no plausible biological mechanism by which antiperspirants could cause breast cancer. Indeed, in the past, national cancer charities and other authorities (including Breakthrough Breast Cancer and Cancer Research UK) have seen false allegations as diverting attention away from taking action on those factors known to be associated with a risk of breast cancer, such as smoking and poor diet.
It has also been suggested that aluminium may be able to mimic oestrogen. The strength of any such effect would be extremely low and only detectable under experimental conditions that cannot apply to real life, and there is no evidence that this can harm human health. Many substances have the ability to mimic oestrogen - and these are found at much higher concentrations in the foods we eat. In practice, just because something has the potential to mimic a hormone (in this case oestrogen), it does not mean that it can cause harm to human health. Remember, aluminium is the third most common element in the earth's crust and most of what we absorb comes from food and drink.
Aluminium and Alzheimer's disease
Some researchers have speculated that there could be a link between aluminium and Alzheimer's disease; however, there is no proof that such a relationship exists. There has been a lot of research into this area over the past 40 years. In 1997, the World Health Organisation said that it had found no evidence that aluminium was a health risk for healthy people who were in contact with aluminium because of their jobs, and there was no evidence that aluminium was a primary cause of Alzheimer's disease. Also, epidemiological studies show no unusual incidence of Alzheimer's disease in persons working in aluminium mines or smelters where they could be expected to inhale the substance in large amounts.
The overwhelming medical and scientific opinion is that the current findings do not convincingly demonstrate a causal relationship between aluminium and Alzheimer's disease.
You can read more about this from the Alzheimers Society website.