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You've got a rash. You quickly rule out the usual suspects: You haven't been gardening or hiking or even picnicking, so it's probably not a plant irritant such as poison ivy or wild parsnip; likewise, it's probably not chiggers or ticks carrying Lyme disease; and you haven't been swimming in a pond, which can harbor the parasite that causes swimmer's itch.
Your rash may be coming from two hard-to-escape realities of summer: heat and sun.
Let's consider heat first. Heat rash occurs when the sweat glands are blocked by tiny blisters — they resemble small beads of sweat — that form on the skin. Heat rash, called miliaria by doctors, can cause the skin to redden if the glands are blocked in a deeper layer of the skin. The skin can feel irritated and itchy, giving rise to another common term for the syndrome — prickly heat.
The chain of events is the same in heat rash as in eczema. They both start with staphylococcus bacteria living on the skin, says Herb Allen, a dermatologist and researcher at Drexel University in Philadelphia. "It's normal flora," he says. "It's on everybody."
Sometimes the bacteria produce a substance that scientists call biofilm. "It used to be called slime," Allen says. "It coats and protects bacteria." Biofilm can also clog up sweat glands.
What makes staph bacteria produce biofilm? Salt and water — or sweat. The bacteria sense the salty wetness as a dangerous environment and throw out slime in defense. The slime can block your sweat glands and trigger a rapid immune response that causes an itchy rash.
This sequence — you sweat and then your sweat glands clog — is more likely to occur, Allen says, "when you have too much clothing on — or too-tight clothes — or have been lying still on hospital bedsheets. There's no air flow."
Fresh air, a breeze, air conditioning and fan-circulated air all help sweat evaporate and prevent heat rash. "Adequate ventilation is the key," says Mary Sheu, medical director of the Johns Hopkins Dermatology & Cosmetic Center. To prevent miliaria, wear lightweight clothing — and less of it, Allen says.
Anyone can get heat rash. If you do, a heavy-duty moisturizer, especially one containing lanolin, can provide relief, Allen says. Calamine lotion or cortisone cream can also help.
As for rashes caused by sun, polymorphous light eruption (PMLE) is the most common culprit, affecting about 10 to 20 percent of people in the Unites States.
While every type of skin type can get sunburned, PMLE is an allergic reaction to the sun's ultraviolet rays. Its form varies — it can produce an itchy rash or blisters or raised red blotches — and its severity varies widely. The reaction can emerge suddenly in one place, such as on arms exposed to the sun that first warm day in April.
"The condition tends to occur in the spring or early summer," Sheu says. "It's too much sun, too quickly." So when exposing your winter-hidden skin parts to the sun, go slow. The best protection against PMLE is avoiding exposure to the sun altogether. Next best is wearing clothes that offer sun protection.
Sunscreen products help, but they can't entirely prevent PMLE. "Sunscreens are pretty good at blocking short UV rays, the UVB rays that cause sunburn," Allen says. "But they're not so good at blocking the long UV rays, the UVA rays. Even the best sunscreens block only 40 to 50 percent of UVA rays."
Some sunscreens are better than others. Products labeled "broad-spectrum" are formulated to guard against both UVA and UVB rays. Those containing zinc or titanium oxide — these are known as physical blockers — are the best bet for people susceptible to PMLE reactions, Allen says.
A recent study tested sunscreens on 15 women with a history of PMLE; for each woman, four patches of skin were monitored. No reactions were seen after repeated UV exposure of patches lathered with a dose of a high-UVA-protection sunscreen. When half that amount was applied, five reactions occurred. (The American Dermatology Society recommends one ounce of sunscreen, the amount you can hold in your palm, to cover your whole body.)
With a low-UVA-protection sunscreen, 11 and 12 PMLE reactions were seen when applied at the full dose and half that, respectively.
If you do get bothersome PMLE reactions, steroid creams offer some relief. In severe cases, phototherapy — periodic, gradual exposure to UV light — can harden the skin to the reaction.
"A lot of people aren't aware of PMLE," Sheu says. "They may call it heat rash — which is kind of a catch-all term."
The main difference? PMLE, like sunburn, occurs on sun-exposed skin. Heat rash occurs where the skin is covered.