Why extreme heat may call for using a different sunscreen

A man applies sunscreen at a park during a heatwave affecting the US Northeast in New York City, US, Jun 20, 2024. REUTERS

Sunscreens are not all the same, and with temperatures reaching record highs, you may want to reconsider which one you are using, experts say.

The two major types – mineral and chemical – handle the sun's ultraviolet (UV) rays differently, and during extremely hot weather, those differences can matter. Here's what you need to know:

HOW DO SUNSCREENS WORK?

Mineral sunscreens contain zinc oxide or titanium dioxide. They create a barrier that reflects UV light before it penetrates the skin. Because mineral sunscreens aren't absorbed, older formulations often had a greasy feel and a white appearance.

Newer formulations, made with mineral nanoparticles, "rub into the skin beautifully," said Dr Jacqueline Watchmaker, a dermatologist in Scottsdale, Arizona and a spokesperson for the American Academy of Dermatology (AAD).

People who do not like the thicker texture of mineral sunscreens often use chemical sunscreens in creams or sprays. The ingredients form a thin protective film that absorbs UV rays and changes their structure, converting them into heat before they penetrate the skin.

Ultimately, the chemicals themselves are absorbed into the bloodstream, and health officials say more research is needed to understand the safety impacts of long-term use.

WHEN DOES IT MAKE A DIFFERENCE?

Ordinarily, "the best type of sunscreen is the one you will use again and again," the AAD advises on its website.

The group recommends use of any water-resistant sunscreen, with a Sun Protection Factor (SPF) of 30 or higher, that offers broad-spectrum protection against UVA rays, which lead to signs of ageing, and UVB rays, which lead to sunburn.

But during extreme heat, when temperatures reach 90 degrees Fahrenheit (32 degrees Celsius) or above, with high humidity, for at least a few days, mineral sunscreens are preferable, according to Watchmaker and Dr Ross Radusky of the Dermatology Treatment & Research Center in Dallas, Texas.

Chemical sunscreens can lose their filtering abilities when exposed to extremely high temperatures.

Extreme heat also means more sweating, and sweat can contribute to itchiness and rashes some people experience from ingredients in chemical sunscreens. Even in the absence of extreme heat, people with sensitive skin should opt for mineral sunscreens, the AAD advises.

APPLY MORE SUNSCREEN, MORE OFTEN

Habits matter too. Many individuals only apply about 20%–50% of the amount of sunscreen needed to achieve the level of SPF protection on the label, the AAD says.

"An adult needs one ounce of sunscreen, which is about a shot glass full," with double layers applied to areas exposed to the most sun such as the face, chest and shoulders, Watchmaker said.

Sunscreen should be applied to dry skin 15 minutes before going outdoors and then re-applied at least every two hours, and after swimming or sweating, the AAD says.

KEEP SUNSCREEN CONTAINERS OUT OF THE HEAT

Eight hours of exposure to temperatures of 86 to 140 F (30 to 60 C) can irreversibly alter a chemical sunscreen's physical characteristics, resulting in decreased efficacy, a 2012 study in the Journal of the American Academy of Dermatology found.

If sunscreen must be left in hot vehicles or in the sun, pack it in a cooler. You can tell if sunscreen may have been exposed to extreme heat if its components have started to separate.

Experts caution about ordering sunscreen online in the summer, especially in hot climates, because of the potential for degradation if left in a hot mailbox.