You probably suffer from contact urticaria
If you always get wheals and itchiness in places where your skin comes into contact with specific materials or substances, than you likely suffer from contact urticaria. The most common form of contact urticaria is the urticaria that comes after touching stinging nettles.
The histamine and acetylcholine contained in the stinging hairs of the plant are released upon contact and cause a standard burning wheal. This reaction by the skin is completely normal and harmless, and can be triggered in any human. Urticarial skin changes behave in exactly the same way following insect stings: hence contact urticaria is neither illness- nor allergy-related, but rather is a normal reaction by the skin.
The reaction is different when allergic contact urticaria is involved. Numerous natural and artificial materials can cause an allergy that then expresses itself through contact urticaria. Allergies to foods are of particular importance here. If a food allergy develops into contact urticaria, then the handling of foods leads to wheal formation on the places of contact, meaning that the hands are often affected during preparation or -- much more seriously -- the mucous membranes during eating. Food allergies expressed through contact urticaria can be very dangerous due to potential swelling in the throat area, and under some circumstances can even trigger anaphylaxis. These food allergies are often related to carrots, celery or other vegetables, apples, peanuts and hazelnuts or stone fruits. Where a contact urticaria from foodstuffs is suspected, extensive allergy testing must be performed.
Contact with animal hair can also lead to allergic wheal formation. Dog and cat hair are the prime culprits. In many cases this unfortunately dictates the removal of the animal from the household, although in some cases it may be sufficient to make changes to the way it is kept.
Latex and latex allergies can also be a cause of contact urticaria. Because no causal treatment is possible for latex allergies, all contact with latex materials must be strictly avoided–often not an easy task.
In many cases perfumes, shower gels or cosmetics are thought to be responsible for wheals and itchiness. That would only be the case if the substance leads to wheals and itchiness very soon after its application, and only at the spots where it came into contact with the skin. Wheals must not form if the substance is not applied. The same applies for detergents and fabric softeners as well as medicinal creams and ointments.
In very rare instances contact with water can lead to contact urticaria. It is not possible to be allergic to water, since humans are comprised 70% of it. Yet it is possible that the water acts as a solvent for allergens on or within the skin that cannot penetrate the skin when in a dry state.
If your wheals cannot be provoked through the triggers mentioned here, then it must be examined whether the skin manifestations here are in fact wheals, as described in the section What is urticaria on this website, or some other skin condition.
If you are certain that the skin manifestation in question involves wheals that cannot be safely and repeatedly provoked through any of the previously discussed triggers, but you nevertheless have the feeling that the episodes do not occur completely randomly, then it could involve a case of spontaneous urticaria. There are various triggers that can cause an episode, but do not always do so reliably. This generally involves drugs (principally pain medicine) or foods (including those that are generally described as "healthy"). More information on the clinical picture of that disease is found in the section Spontaneous acute urticaria if the symptoms have existed for less than 6 weeks, and in the section Spontaneous chronic urticaria if the symptoms have existed for more than 6 weeks.
Emotional strains, stress and heat are non-specific triggers of urticaria; those factors can lower the threshold for an approaching urticaria episode, leading to quicker or more pronounced wheal formation. Stress and emotional strain can also lead to an altered self-perception, meaning that wheals and itchiness seem even more threatening than they really are – which leads to more stress, and so on.
Wheals that do not itch or that only itch a little or that remain in the same place for more than 24 hours, or long lasting wheals that react only to cortisone preparations, must be discussed on an individual basis with a dermatologist. In those cases a skin sample must be taken. The actual duration of a wheal should be checked ahead of time through colour marking.