According to the hygiene hypothesis, when children are brought up exposed to allergens in the environment at a young age, their immune system is more likely to tolerate them, while children brought up in a modern "sanitary" environment are less likely to be exposed to those allergens at a young age, and, when they are finally exposed, develop allergies. There is some support for this hypothesis with respect to AD. Those exposed to dogs while growing up have a lower risk of atopic dermatitis. There is also support from epidemiological studies for a protective role for helminths against AD. Likewise children with poor hygiene are at a lower risk for developing AD, as are children who drink unpasteurised milk.

In a small percentage of cases, atopic dermatitis is caused by sensitization to foods. Also, exposure to allergens, either from food or the environment, can exacerbate existing atopic dermatitis. Exposure to dust mites, for example, is believed to contribute to one's risk of developing AD. A diet high in fruits seems to have a protective effect against AD, whereas the opposite seems true for fast foods. Atopic dermatitis sometimes appears to be associated with celiac disease and non-celiac gluten sensitivity, and the improvement with a gluten-free diet indicates that gluten is a causative agent in these cases.

There is no good evidence that a mother's diet during pregnancy, the formula used, or breastfeeding changes the risk. There is tentative evidence that probiotics in infancy may reduce rates but it is insufficient to recommend its use.

People with eczema should not get the smallpox vaccination due to risk of developing eczema vaccinatum, a potentially severe and sometimes fatal complication.

Three main factors play an essential role in the development of chronic hand eczema: excessive contact with water and irritants (cumulative-toxic stress), contact with allergens, and atopic diathesis, which has a genetic component. Individual hand eczema types are identified and named according to the main catalysts involved, i.e. cumulative-toxic, contact-allergenic, or atopic hand eczema. Activities that are stressful for the skin or involve repeated, excessive contact with water or skin-irritating substances at work or home can cause damage to the skin's protective abilities and increase the chances of inflammation. This also applies to prolonged wearing of protective rubber gloves and similar materials, since sweating occurs within these gloves. Disturbance of the skin's protective barrier also facilitates penetration by allergenic substances and promotes the development of contact dermatitis. Contact allergies play a very important role in the development of hand eczema. If the hand is subjected to repeated contact with a substance that leads to an allergy, the skin reacts with signs of inflammation. Numerous people affected by hand eczema also experience skin inflammation on their feet. Often, a contact allergy to an ingredient in shoe leather treatment may be the catalyst. Contact allergies in certain types of employment are especially problematic, particularly if the work involves the handling of allergenic materials, e.g. masonry work or hairdressing.

People allergic to Balsam of Peru may experience a flare-up of hand eczema if they use or consume products that use it as an ingredient.

Severe and chronic eczema qualifies as one of the most frequent occupational illnesses. Patients should therefore be referred to an occupational-dermatological specialist as soon as possible. Patients with a history of neurodermitis, hay fever, or allergic asthma often develop hand eczema. These sicknesses reflect the individual's particular susceptibility or genetic predisposition to react over-sensitively to various environmental influences. This is described as atopy or atopy diathesis. Atopic diathesis is frequently accompanied by genetically conditioned problems with the skin's protective barriers, which causes a weakening of the skin's resistance against irritating substances and moisture, as well as easier penetration by allergens, which may lead to the development of contact allergies.

Hand eczema is a common condition: study data indicates a one-year prevalence of up to 10% in the general population. It is estimated that only 50–70% of people affected consult a doctor. The frequency of severe, chronic and recurrent forms of hand eczema is estimated at 5–7%. Approximately 2–4% of hand eczema patients also report that external (topical) therapy is insufficient.

Several factors adversely affect the long-term prognosis, including the development of the condition prior to the 20th birthday, the severity of initial manifestations, and eczema during childhood. Women, especially those under 30, are more frequently affected than men.

Most cases are well managed with topical treatments and ultraviolet light. About 2% of cases are not. In more than 60% of young children, the condition subsides by adolescence.

The exact causes of dyshidrosis are unknown. In 2013, a randomized, double-blind, placebo-controlled cross-over study by the University Medical Center Groningen reported that dyshydrosis outbreaks on the hands increased significantly among those allergic to house dust mites, following inhalation of house dust mite allergen.

Food allergens may be involved in certain cases. Cases studies have implicated a wide range of foods including tuna, tomato, pineapple, chocolate, coffee, and spices among others. A number of studies have implicated balsam of Peru.

Id reaction and irritant contact dermatitis are possible causes.

With no particular affinity to any particular ethnic group, seen in all age groups and equally amongst males and females, the precise prevalence is not known.

About 1 in 2,000 people are affected in Sweden. Males and females appear to be affected equally.

Common causes of rashes include:

- Food allergy

- Medication side effects

- Anxiety

- Allergies, for example to food, dyes, medicines, insect stings, metals such as zinc or nickel; such rashes are often called hives.

- Skin contact with an irritant

- Fungal infection, such as ringworm

- Balsam of Peru

- Reaction to vaccination

- Skin diseases such as eczema or acne

- Exposure to sun (sunburn) or heat

- Friction due to chafing of the skin

- Irritation such as caused by abrasives impregnated in clothing rubbing the skin. The cloth itself may be abrasive enough for some people

- Secondary syphilis

- Poor personal hygiene

Uncommon causes:

- Autoimmune disorders such as psoriasis

- Lead poisoning

- Pregnancy

- Repeated scratching on a particular spot

- Lyme Disease

- Scarlet fever

Many contact sensitizers or irritants are known to cause contact dermatitis superimposed on nummular dermatitis. Studies have implicated nickel, cobalt, chromate, and fragrance as likely culprits. Xerosis, or dehydration of skin is also a likely cause. Infection with "Staphylococcus aureus" bacteria or "Candida" may also play a role.

In adults, the prevalence of IgE sensitization to allergens from house dust mite and cat, but not grass, seem to decrease over time as people age. However, the biological reasons for these changes are not fully understood.

Symptoms are thought to be the result of histamine being released by mast cells on the surface of the skin. Due to the lack of antigens, histamine causes the skin to swell in affected areas. If the membrane that surrounds the mast cells is too weak it will easily and rapidly break down under physical pressure, which will therefore cause an allergic-like reaction.

Symptoms can be caused or induced by

- stress

- tight or abrasive clothing

- watches

- glasses

- heat

- cold

- anything placing pressure on exposed skin

- infection

The underlying cause of dermatographism is not known, and can last for many years without relief. The condition may subside and be effectively cured; however, it is often a lifelong ailment. It is not a life-threatening disease and is not contagious.

Dermographism may occur in Mastocytosis (systemic mast cell proliferation).

Other rashes that occur in a widespread distribution can look like an id reaction. These include atopic dermatitis, contact dermatitis, dyshidrosis, photodermatitis, scabies and drug eruptions.

The prevalence of nummular dermatitis in the United States is approximately 2 per 1,000. It is considered a disease of adulthood, for it is rare in children.

One way to treat xerotic eczema is to avoid ing the affected area and to apply anti-itch or moisturizing lotion frequently.

A study published in 2005 found positive results from soaking the affected area in water for twenty minutes and then applying mid- to high-strength corticosteroid ointment.

Rosin, the material commonly used to wax string instruments is known to cause allergic contact dermatitis in musicians. Nickel, a metal found in musical instruments causes allergic contact dermatitis on the fingers and hands of string instrumentalists and in the lip and neck of wind instrumentalists. Wind instrumentalists with lip and neck infection should switch to gold or plastic mouthpieces if allergic dermatitis occurs. (R)-4-methoxydalbergione present in rosewood may cause allergic contact dermatitis in violinists. Cane reed (causing chelitis in saxophone players), propolis (a wax used to close structural gaps in musical instruments), paraphenylenediamine (used to polish musical instruments) and potassium dichromate (tanning agent to the skin of the harp) also cause allergic contact dermatitis in musicians.

Dermatographic urticaria manifests as an allergic-like reaction, in general a warm red wheal (welt) to appear on the skin. It can often be confused with an allergic reaction to the object causing the scratch, when in fact it is the act of being scratched that causes a wheal to appear. These wheals are a subset of [urticaria] (hives) that appear within minutes, in some cases accompanied by itching. The first outbreak of urticaria can lead to others on body parts not directly stimulated, scraped, or scratched. In a normal case, the swelling will decrease with no treatment within 15–30 minutes, but, in extreme cases, itchy red welts may last anywhere from a few hours to days.

There is a strong genetic predisposition toward atopic allergies, especially on the maternal side. Because of the strong familial evidence, investigators have tried to map susceptibility genes for atopy. Genes for atopy (C11orf30, STAT6, SLC25A46, HLA-DQB1, IL1RL1/IL18R1, TLR1/TLR6/TLR10, LPP, MYC/PVT1, IL2/ADAD1, HLA-B/MICA) tend to be involved in allergic responses or other components of the immune system. C11orf30 seems to be the most relevant for atopy as it may increase susceptibility to poly-sensitization.

One of the most common areas of the body associated with irritation is the vagina. Many women complain of an itch, dryness, or discharge in the perineum at some point in their lives. There are several causes of vaginal irritation including fungal vaginitis (like candida) or trichomoniasis. Often, herpes simplex infection of the mouth or genitalia can be recurrent and prove to be extremely irritating.

Sometimes, the irritation can be of the chronic type and it can be so intense that it also causes painful intercourse. Aside from infections, chronic irritation of the vagina may be related to the use of contraceptives and condoms made from latex. The majority of contraceptives are made of synthetic chemicals which can induce allergies, rash and itching. Sometimes the lubricant used for intercourse may cause irritation.

Another cause of irritation in women is post menopausal vaginitis. The decline in the female sex hormones leads to development of dryness and itching in the vagina. This is often accompanied by painful sexual intercourse. Cracks and tears often develop on outer aspects of the labia which becomes red from chronic scratching. Post menopausal vaginitis can be treated with short term use of vaginal estrogen pessary and use of a moisturizer.

Frequent, chronic contact of instruments to skin may make it callous by the thickening of stratum corneum. Use of 'thumb position' in cellists may cause callosity of left thumb. Garrod's pads are seen on the dorsal left second and third fingers over the proximal interphalangeal joints in violinists. Drummer's digit is the callosity seen on the lateral phalynx of the left finger. Callosities need treatment only when they are excessive or symptomatic.

Gastritis or stomach upset is a common irritating disorder affecting millions of people. Gastritis is basically inflammation of the stomach wall lining and has many causes. Smoking, excess alcohol consumption and the use of non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, account for the majority of causes of gastritis. In some cases, gastritis may develop after surgery, a major burn, infection or emotional stress. The most common symptoms of gastritis include sharp abdominal pain which may radiate to the back. This may be associated with nausea, vomiting, abdominal bloating and a lack of appetite. When the condition is severe it may even result in loss of blood on the stools. The condition often comes and goes for years because most people continue to drink alcohol or use NSAIDs. Treatment includes the use of antacids or acid neutralizing drugs, antibiotics, and avoiding spicy food and alcohol.

A rash is a change of the human skin which affects its color, appearance, or .

A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell, and may be painful.

The causes, and therefore treatments for rashes, vary widely. Diagnosis must take into account such things as the appearance of the rash, other symptoms, what the patient may have been exposed to, occupation, and occurrence in family members. Rash can last 5 to 20 days, the diagnosis may confirm any number of conditions.

The presence of a rash may aid diagnosis; associated signs and symptoms are diagnostic of certain diseases. For example, the rash in measles is an erythematous, morbilliform, maculopapular rash that begins a few days after the fever starts. It classically starts at the head, and spreads downwards.

Xerotic eczema (also known as "Eczema craquelé", "Pruritus hiemalis", "Asteatotic eczema", "Winter itch", "Desiccation dermatitis," and "Winter eczema") is a form of eczema that is characterized by changes that occur when skin becomes abnormally dry, itchy, and cracked. Lower legs tend to be especially affected, although it can appear in the underarm area as well.

Xerotic eczema is common in elderly people, though it is not uncommon for people in their 20s. It can appear in red, bumpy, pimple-like irritations. Shaving can cause it to become inflamed.

Any dermatitis may heal leaving pale skin, as may excessive use of corticosteroid creams used to treat episodes of eczema. The hypopigmentation is due to both reduced activity of melanocytes with fewer and smaller melanosomes.

The condition is most often seen in children between the ages of 3 and 16 years and is more common in males than females. However adults can also suffer from this disease.

It may occur more frequently in lighter-skinned patients, but is more apparent in those with darker complexions.

Up to a third of US school children may at some stage have this condition. Single-point prevalence studies from India have shown variable rates from 8.4%,

to 31%.

Other studies have shown prevalence rates in Brazil of 9.9%,

Egypt 13.49%,

Romania 5.1%,

Turkey 12% where higher rates were seen in those with poor socioeconomic conditions,

and just 1% in school children in Hong Kong.