Heat Rash?

I have moved to a hot, dry environment (Boulder,Colorado) after living in New England.   It has been more than a month, but the heat still bothers me.   Recently I got what I think is a heat rash on the insides of my arms.  I have tried to keep cool but my job requires that I be outside, where I get sweaty fast.   My rash has spread over my arms, little pimply like itchy bumps that get much bigger if I itch, which I haven't done much of.   Now it is on my knees and thighs and seems to be spreading even though I am inside with the AC on.   I have tried using Caladryl, which makes it less itchy, but it seems to still be spreading.  I also do not have health insurance, so any help you can give me would be great.

[Dr_Aparna]

Hi,
This could be a form of what is called a physical allergy. The body reacts to non-proteinaceous substances such as heat, cold etc.

Cholinergic urticaria is a subcategory of physical urticaria ( hives) that is a skin rash brought on by a hypersensitive reaction to body heat. Symptoms follow any stimulus to sweat such as exercise (sometimes called exercise-induced urticaria), heat from the sun (which could also indicate solar urticaria), saunas, hot showers.

The visible hives (sometimes called heat bumps) appear as a multitude of small 2-3 mm welts typically surrounded by patches of red skin. The affected area will often feel warm and can be extremely itchy or exhibit a burning sensation. Typically the rash occurs on the upper trunk and the arms but can appear on other parts of the body. The rash may be worse in areas where clothing restricts the skin's ability to cool itself.

The rash typically develops within a few minutes of a rise in body temperature but can take longer to appear visibly on the skin. The visible rash is often preceded by a general warming of the skin or itchiness. The hives last from a half an hour to several hours.

All urticarias are caused by an elevated histamine release by the body's mast cells. With cholinergic urticaria the exact triggering mechanism for this response is unknown, but it is assumed to be related to the body's thermoregulatory response.

Since an attack can often be felt coming on, it can sometimes be halted by rapid cooling, such as applying cold water or an ice pack to the skin.
Drug treatment is typically in the form of antihistamines such as loratadine (Claritin), hydroxyzine, cetirizine. Try applying Calamine lotion or menthol-based creams to soothe the itchy skin.

ref:http://en.wikipedia.org/wiki/Cholinergic_urticaria