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.

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.

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.

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.

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.

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.

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.

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).

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.

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

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.

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

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.

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.

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.

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 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.

Individuals who smoke or are exposed to smog or other airborne pollutants can develop a condition known as COPD. In this disorder, there is constant irritation of the breathing tubes (trachea) and the small airways. The constant irritation results in excess production of mucus which makes breathing difficult. Frequently, these individuals wake up in the morning with copious amounts of foul smelling mucus and a cough which lasts all day. Wheeze and heavy phlegm are common findings. COPD is a lifelong disorder and there is no cure. Eventually most people develop recurrent pneumonia, lack any type of endurance, and are unable to work productively. One of the ways to avoid chronic bronchitis is to stop or not smoke.

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.

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.

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.

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.

This is a skin disorder characterized by a self-perpetuating scratch-itch cycle:

- It may begin with something that rubs, irritates, or scratches the skin, such as clothing.

- This causes the person to rub or scratch the affected area. Constant scratching causes the skin to thicken.

- The thickened skin itches, causing more scratching, causing more thickening.

- Affected area may spread rapidly through the rest of the body.

Many hypothesize LSC has a psychosomatic origin. Those predisposed to itch as a response to emotional tensions may be more susceptible to the itch-scratch cycle. It may also be associated with nervousness, anxiety, depression, and other psychological disorders. Many people with LSC are aware of the scratching they do during the day, but they might not be aware of the scratching they do in their sleep. LSC is also associated with atopy, or atopic dermatitis (eczema).

Lichen simplex chronicus (LSC) (also known as "Neurodermatitis") is a skin disorder characterized by chronic itching and scratching. The constant scratching causes thick, leathery, darkened, (lichenified) skin. This condition is associated with many factors, including the scratch-itch cycle, psychological stressors, and atopy. LSC is more common between ages 35 and 50 and is seen approximately twice as often in women compared to men.