A number of top brand sunscreen products risk causing cancer if they're left on a shelf for too long, a recent clinical study finds, revealing they release toxic chemicals over time.

Scientists from France and the US studied a number of randomly selected brand name sunscreen products containing sun filter' chemical octocrylene. Octocrylene is a common ingredient of sunscreens as it absorbs UVB radiation, which is responsible for sunburn and skin cancer, but is controversial as it is a potential allergen and isn't biodegradable. It is already banned in a number of places.

The team from Sorbonne University found that octocrylene degrades into the carcinogenic chemical benzophenone when left on the shelf for more than a year.

Study authors say the manufacture of products containing the chemical octocrylene should be banned until the industry can prove 'beyond doubt' that it is safe. Among them is benzophenone, which is a bioaccumulative 'mutagen, carcinogen and endocrine disruptor' which can lead to cancer.

An endocrine disruptor is a chemical that can interfere with the messengers in the body that regulate the circulatory system and can result in cancerous tumors.

Dermatologist, Dr Sharyn Laughlin, involved in this study, said she was gravely concerned regarding the far-reaching consequences of this new research. "Benzophenone is still the worst offender in causing contact dermatitis, and may induce uticaria and anaphylaxis. Absorption of both chemicals through the skin poses a potential threat to toxicities and diseases of other organs."Benzophenone is already banned in food products and food packaging in the US and from personal care products including sunscreens and moisturizers."

The chemical octocrylene is commonly used in sunscreen as it acts as an 'organic sun filter', protecting the skin from UVB radiation.

However, it is a controversial product that has already been banned in the US Virgin Islands and Marshall Islands. This is in part due to its impact on the marine environment and that it is non-biodegradable.

In coral it accumulates and can result in bleaching and death. Finding out that concentrations of the harmful chemical increase in sunscreen containing octocrylene the older it gets is 'very worrying,' the team said.

To test their theory that benzophenone is produced over time from octocrylene, the team selected a random number of creams directly from stores after looking for the best sellers. They then artificially aged them over a six-week period to replicate conditions found in a normal family home over the course of an entire year.

Benzophenone developed in increasingly high concentrations the older the product got - so buying sunscreen to use on vacation and then leaving it for a year before using it again could be harmful.

Both octocrylene and benzophenone are readily absorbed into the skin.

Dermal absorption of benzophenone into the body may exceed 70%, based on topical studies conducted by Howard Maibach and colleagues in the 1990s.

In mammalian-model studies, benzophenone exposure quickly gave rise to liver cancers and lymphomas.

Benzophenone can act as a photo-mutagen; in the presence of light, it increases the rate of DNA lesions, thus increasing the risk of skin cancers. Benzophenone is also an endocrine disruptor, affecting thyroid function as well as inducing anti-androgenic activity, delaying testicular development and causing anatomic difficulties with female reproductive organs.

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