Mixing different sunscreens could almost obliterate part of the protection offered from UV rays, a study suggests.
Experts found combining 'chemical' creams with 'mineral' ones which contain zinc oxide reduced the barrier they provide against the sun by up to 91 per cent.
Researchers today issued a warning to consumers about the potential interaction, and said the chemical can also be found in SPF products such as moisturisers and make-up.
Independent experts today emphasised the importance of creams and SPF-based items for blocking the sun's powerful rays.
Sunscreen is one of the best ways to both enjoy the sun and keep yourself safe from a nasty burn and long term risk of skin cancer.
Chemical sunscreens contain substances which absorb UV rays before they reach the skin, diffusing them in a chemical reaction.
These products, also known as organic or synthetic sunscreens, are absorbed by the body.
Whereas a mineral sunscreen, made of ingredients such as zinc oxide or titanium dioxide, provides a physical barrier preventing UV rays from hitting the skin.
Mineral sunscreens have been advertised as a better choice to those with sensitive skin, such as children, who can experience irritation from chemicals.
Researchers have found mixing different types sun protection products could dramatically reduce some of the protection offered
Academics at the universities of Oregon and Leeds tested different SPF 15 sunscreen mixtures made of ingredients approved for use in the US and EU.
These are also called organic or synthetic sunscreens and are the most common type of sunscreen.
They work by absorbing UV light and releasing it as heat.
The following chemicals can be found found in chemical sunscreens: Oxybenzone, Avobenzone, Octisalate, Octocrylene, Homosalate, Octinoxate
Chemical sunscreen is absorbed into the skin and can sometimes cause irritation.
They need to be applied 20 minutes before sun exposure and need to be applied regularly.
Example products: Sun Bum, Nivea, Soltan
Also known as physical sunscreens, these work by absorbing and then scattering or deflecting the UV radiation in addition to absorbing the UV light and releasing as heat.
Two substances are used in mineral sunscreens: titanium dioxide, zinc dioxide.
Mineral sunscreens offer immediate protection from the sun however can be washed off with sweat and water.
They are less irritable to skin but usually require larger amounts to achieve the same protection as chemical sunscreens.
Example brands: Alba Botanica, Avène, Hawaiian Tropic
They tested the mixtures on their levels of UVA protection, which is one element of UV radiation, the other being UVB.
Though UVB is responsible for most skin cancers, both types of UV can potentially cause cancers.
The researchers were particularly concerned by one mixture, the formula of which represented the majority of sun cream mixtures used in the EU and US.
They found it suffered a 90 per cent drop in UVA protection when mixed with 6 per cent zinc oxide after two hours exposure.
In comparison, when the formula was exposed to UV for two hours without the zinc oxide it only suffered a 16 per cent loss in protective ability.
The scientists found that the zinc oxide, when exposed to light, degraded the other UV absorbing chemicals in the mixture — reducing its protective qualities.
Co-author of the study Richard