An elevation in core body temperature resulting in sweating causes the rash in cholinergic urticaria. Common triggers include:
Eating spicy foods
Patients with cholinergic urticaria often have another associated physical urticaria, such as dermographism, cold urticaria or pressure urticaria.
The rash occurs within a few minutes of sweating, and can take between 30 minutes to more than one hour to resolve. Typical signs and symptoms of the rash include:
Itching, burning, tingling, warm sensation, followed by the appearance of many small weals with surrounding bright red flares.
Sometimes the tiny weals coalesce together into a large plaque.
Rarely, patients with cholinergic urticaria can experience more severe reactions such as anaphylaxis.
People with allergic type conditions, such as asthma, rhinitis, atopic dermatitis (eczema), have a greater tendency to develop cholinergic urticaria. This condition is more common in men than women and seems to first appear in people aged between 10 and 30 years and persists for a number of years before becoming less severe or completely resolving.
Once the causative agent of the rash is identified, it may be possible to avoid situations that trigger it. In your case, you should obviously avoid hot baths and only use warm water while bathing. However, stopping sweating may not always be possible, mainly in warm climates and during exercising. Sometimes rapid cooling can prevent an attack. For most cases, oral antihistamine such as cetirizine can be helpful in preventing the condition from arising. Beta-blockers such as propranolol have also been reported to be useful.