One proposed hypothesis for the cause of multiple chemical sensitivity is immune system dysfunction after being sensitized by a chemical exposure.

Many chemicals have been reported to trigger MCS symptoms. Substances with strong scents are the most commonly reported triggers. These include a variety of cleaning agents, pesticides, perfumes, vehicle exhaust, the products used in barber shops and beauty salons, new carpeting, new furniture, chlorine and fluoride in drinking water, fresh ink, and less commonly wood smoke and secondhand tobacco smoke. Food items reported as triggers include tartrazine (a.k.a. FD&C Yellow #5 or E102), and other azo dyes (in the absence of an allergy), caffeine, and monosodium glutamate.

Depending on whether the salicylate is a component of food or medicine, salicylate intolerance is a form of food intolerance or of drug intolerance.

Salicylate sensitivity is a pharmacological reaction, not a true IgE-mediated allergy. However, it is possible for aspirin to trigger non-allergic hypersensitivity reactions. About 5–10% of asthmatics have aspirin hypersensitivity, but dietary salicylates have been shown not to contribute to this. The reactions in AERD (Samter's triad) are due to inhibition of the COX-1 enzyme by aspirin, as well as other NSAIDs that are not salicylates. Dietary salicylates have not been shown to significantly affect COX-1.

Samter's triad refers to aspirin sensitivity in conjunction with nasal polyps and asthma.

Feline hyperesthesia syndrome is an uncommon but recognized condition in cats, particularly Siamese, Burmese, Himalayan, and Abyssinian cats. It can affect cats of all ages, though it is most prevalent in mature animals. The disease can be somewhat difficult to detect as it is characterized by brief bursts of abnormal behavior, lasting around a minute or two. One of its symptoms is also found in dogs that have canine distemper disease (CD) caused by canine distemper virus (CDV).

An important salicylate drug is aspirin, which has a long history. Aspirin intolerance was widely known by 1975, when the understanding began to emerge that it is a pharmacological reaction, not an allergy.

Allergies to a specific pollen are usually associated with OAS reactions to other certain foods. For instance, an allergy to ragweed is associated with OAS reactions to banana, watermelon, cantaloupe, honeydew, zucchini, and cucumber. This does not mean that all sufferers of an allergy to ragweed will experience adverse effects from all or even any of these foods. Reactions may be associated with one type of food, with new reactions to other foods developing later. However, reaction to one or more foods in any given category does not necessarily mean a person is allergic to all foods in that group.

Although genetic factors govern susceptibility to atopic disease, increases in atopy have occurred within too short a time frame to be explained by a genetic change in the population, thus pointing to environmental or lifestyle changes. Several hypotheses have been identified to explain this increased rate; increased exposure to perennial allergens due to housing changes and increasing time spent indoors, and changes in cleanliness or hygiene that have resulted in the decreased activation of a common immune control mechanism, coupled with dietary changes, obesity and decline in physical exercise. The hygiene hypothesis maintains that high living standards and hygienic conditions exposes children to fewer infections. It is thought that reduced bacterial and viral infections early in life direct the maturing immune system away from T1 type responses, leading to unrestrained T2 responses that allow for an increase in allergy.

Changes in rates and types of infection alone however, have been unable to explain the observed increase in allergic disease, and recent evidence has focused attention on the importance of the gastrointestinal microbial environment. Evidence has shown that exposure to food and fecal-oral pathogens, such as hepatitis A, "Toxoplasma gondii", and "Helicobacter pylori" (which also tend to be more prevalent in developing countries), can reduce the overall risk of atopy by more than 60%, and an increased rate of parasitic infections has been associated with a decreased prevalence of asthma. It is speculated that these infections exert their effect by critically altering T1/T2 regulation. Important elements of newer hygiene hypotheses also include exposure to endotoxins, exposure to pets and growing up on a farm.

Hyperesthesia (or hyperaesthesia) is a condition that involves an abnormal increase in sensitivity to stimuli of the sense. "When a non-noxious stimulus causes the sensation of pain the area will be termed hyperaesthetic". Stimuli of the senses can include sound that one hears, foods that one tastes, textures that one feels, and so forth. Increased touch sensitivity is referred to as "tactile hyperesthesia", and increased sound sensitivity is called "auditory hyperesthesia". Tactile hyperesthesia may be a common symptom of many neurologic disorders such as herpes zoster, peripheral neuropathy and radiculopathies. In 1979, and then in 1994, Merskey, Bogduk, Noordenbos, Devor and others (a subcommittee of International Association for the Study of Pain) proposed, instead of hyperaesthesia, the concept of allodynia, meaning "other pain", defined as a pain resulting from a stimulus that does not normally provoke pain.

In psychology, Jeanne Siaud-Facchin uses the term by defining it as an "exacerbation des sens" that characterizes gifted children (and adults): for them, the sensory information reaches the brain much faster than the average, and the information is processed in a significantly shorter time.

There is a great deal of conflicting information regarding the inclusion of oats in a gluten-free diet. Although cross-contamination in the field and during processing partially explains the different reactions that celiacs can have to oats, a recent study indicates that there are also different amounts of avenin present in different cultivars of oat. The G12 antibody used in the study is currently the only one that can reliably distinguish between varieties of oat. Previous studies have indicated both children and adult coeliacs are largely tolerant of oats. Other studies have followed both children and adults for one, two, and five years on the "uncontaminated" oat containing gluten-free diet. These studies failed to show significant changes in intestinal morphology indicative of a relapse of celiac disease. Anti-gliadin and reticulin antibodies as well as numbers of intraepithelial lymphocytes (IELs) did not differ significantly between oat-eating celiacs and non-oat-eating controls in remission. Invitro tests that are sensitive to wheat gluten found that tryptic peptides of avenin could not induce EMA production in supernatant fluid from cultured duodenal mucosa specimen from celiac patients.

Algorithms that successfully predict T cell stimulatory peptides in gluten identified many similar peptides in hordeins and secalins, but not in oat avenins.

The Canadian Celiac Association suggests that adults can consume up to 70g of oats per day, and children up to 25g. However, two studies indicated that celiac adults could consume 93 grams (3.3 ounces) of oats per day. There is no evidence that oats can trigger GSE, only that in a small number of celiacs disease can be sustained or reinitiated by oats once triggered by wheat. A recent paper examining the IEL levels of celiac patients in remission showed a significantly higher number of IELs in oat-eating celiacs. In addition, antibodies to avenin remain low as long as the diet is gluten-free, but higher anti-avenin antibodies can increase with a diet containing wheat.

Some coeliacs respond adversely to oats. Estimates range from 0.5 to 20% of the GSE population. With coeliac disease, non-compliance in attempting to achieve normal intestinal morphology is a risk factor for refractory disease and cancer.

Prevention includes avoiding exposure to the sun and wearing sun block on the affected area.

- Cover up: wear long sleeves, slacks, and a wide-brimmed hat whenever harsh exposure is probable

- Avoid chemicals that may trigger a reaction

- Wear sunscreen at least factor 30 with a high UVA protection level

- Wear gloves and/or remain indoors after handling fruits or plants which increase sensitivity to light

Studies on farmers with grain dust allergy and children with atopy dermatitis reveal that oat proteins can act as both respiratory and skin allergens. Oat dust sensitivity in farms found 53% showed reactivity to dust, second only to barley (70%), and almost double that of wheat dust. The 66 kDa protein in oats was visualized by 28 out of 33 sera (84%). However, there was evident non-specific binding to this region and thus it may also represent lectin-like binding. IgA and IgG responses, meanwhile, like those seen to anti-gliadin antibodies in celiac disease or dermatitis herpetiformis, are not seen in response to avenins in atopic dermatitis patients. Food allergies to oats can accompany atopy dermatitis. Oat avenins share similarities with γ and ω-gliadins of wheat — based on these similarities they could potentiate both enteropathic response and anaphylactic responses. Oat allergy in gluten-sensitive enteropathy can explain an avenin-sensitive individual with no histological abnormality, no T-cell reaction to avenin, bearing the rarer DQ2.5"trans" phenotype, and with anaphylactic reaction to avenin.

Despite comparisons to panic attack, investigators have identified ataque de nervios as a separate syndrome with measured differences in anxiety sensitivity and types of attacks.

The allergic diseases—hay fever and asthma—have increased in the Western world over the past 2–3 decades. Increases in allergic asthma and other atopic disorders in industrialized nations, it is estimated, began in the 1960s and 1970s, with further increases occurring during the 1980s and 1990s, although some suggest that a steady rise in sensitization has been occurring since the 1920s. The number of new cases per year of atopy in developing countries has, in general, remained much lower.

OAS produces symptoms when an affected person eats certain fruits, vegetables, and nuts. Some individuals may only show allergy to only one particular food, and others may show an allergic response to many foods.

Individuals with an allergy to tree pollen may develop OAS to a variety of foods. While the tree pollen allergy has been worked out, the grass pollen is not well understood. Furthermore, some individuals have severe reactions to certain fruits and vegetables that do not fall into any particular allergy category. In recent years, it has also become apparent that when tropical foods initiate OAS, allergy to latex may be the underlying cause.

Because the allergenic proteins associated with OAS are usually destroyed by cooking, most reactions are caused by eating raw foods. The main exceptions to this are celery and nuts, which may cause reactions even after being cooked.

Some conditions that are associated with hyperacusis include:

- Acoustic shock

- Adverse drug reaction

- Anxiety

- Autism spectrum

- Lyme disease

- Migraine

- Ménière's disease

- Endolymphatic hydrops

- Multiple Sclerosis

- Noise-induced hearing loss

- Posttraumatic stress disorder

- Severe head trauma

- Superior canal dehiscence syndrome (SCDS)

- Systemic lupus erythematosus (SLE)

- Tay–Sachs disease

- Williams syndrome

Reported aspects of the syndrome include uncontrollable screaming or shouting, crying, trembling, sensations of heat rising in the chest and head, dissociative experiences, and verbal or physical aggression. The reaction is usually associated with a stressful event relating to the family, although it is not specifically defined as arising from such occurrences.

Milk allergy typically presents in the first year of life. The majority of children outgrow milk allergy by the age of ten years. One large clinical trial reported resolutions of 19% by age 4 years, 42% by age 8 years, 64% by age 12 years, and 79% by 16 years. Children are be able to tolerate milk as an ingredient in baked goods relative to liquid milk. Resolution was more likely if baseline serum IgE was lower, or if IgE-mediated allergy was absent so that all that was present was cell-mediated, non-IgE allergy.

People with confirmed cow's milk allergy may also demonstrate an allergic response to beef, moreso to rare beef versus well-cooked beef. The offending protein appears to be bovine serum albumin. This is not the same beef allergy that is seen primarily in the southeastern United States, triggered by being bitten by a Lone Star tick.

Milk allergy has consequences. In a U.S. government diet and health surveys conducted in 2007-2010, 6,189 children ages 2-17 years were assessed. For those classified as cow's milk allergic at the time of the survey, mean weight, height and body-mass index were significantly lower than their non-allergic peers. This was not true for children with other food allergies. Diet assessment showed a significant 23% reduction of calcium intake and near-significant trends for lower vitamin D and total calorie intake.

Irlen syndrome, occasionally referred to as scotopic sensitivity syndrome (SSS) or Meares-Irlen syndrome, very rarely as asfedia, and recently also as visual stress, is a proposed disorder of vision.

The Irlen Method uses coloured overlays and tinted lenses in the form of glass or contact lenses. The method is intended to reduce or eliminate perceptual processing errors; it is claimed the resultant retiming of visual signals in the brain improves the reading difficulties associated with scotopic sensitivity syndrome.

Incidence and prevalence are terms commonly used in describing disease epidemiology. Incidence is newly diagnosed cases, which can be expressed as new cases per year per million people. Prevalence is the number of cases alive, expressible as existing cases per million people during a period of time. Milk allergies are usually observed in infants and young children, and often disappear with age (see Prognosis), so prevalence of egg allergy may be expressed as a percentage of children under a set age. Milk allergy affects between 2% and 3% of infants in developed countries. This estimate is for antibody-based allergy; prevalence of allergy based on cellular immunity is unknown.

For all age groups, a review of fifty studies conducted in Europe estimated 6.0% for self-reported milk allergy and 0.6% for confirmed. National survey data in the United States collected 2005-2006 showed that from age six and older, the prevalence of serum IgE confirmed milk allergy was under 0.4%.

Antibodies to α-gliadin have been significantly increased in non-celiacs individuals with oral ulceration. Anti-α-gliadin antibodies are frequently found in celiac disease (CD), to a lesser degree CD, but are also found in a subset who do not have the disease. Of people with pseudo-exfoliation syndrome, 25% showed increased levels of anti-gliadin IgA. Other patients that are also at risk are those taking gluten despite having the disorder, or whose family members with CD. In addition patients with autoimmune conditions are also at risk for CD. It has just been found that there is a risk of death in CD. Therefore gluten intake should be limited before or even after the diagnosis. One fourth of people with Sjögren's syndrome had responses to gluten, of 5 that had positive response to gluten, only one could be confirmed as CD and another was potentially , the remaining 3 appear to be gluten-sensitive. All were HLA-DQ2 and/or DQ8-positive.

As possible preventative interventions, the American National Cancer Institute Symptom Management and Health-related Quality of Life Steering Committee recommends continued investigation of several dietary supplements, including glutathione, and intravenous calcium and magnesium, which have shown early promise in limited human trials; acetyl-L-carnitine, which was effective in animal models and on diabetes and HIV patients; and the anti-oxidant alpha-lipoic acid.

In examining the published studies on opioid-induced hyperalgesia (OIH), Reznikov "et al" criticize the methodologies employed on both humans and animals as being far-removed from the typical regimen and dosages of pain patients in the real world. They also note that some OIH studies were performed on drug addicts in methadone rehabilitation programs, and that such results are very difficult to generalize and apply to medical patients in chronic pain. In contrast, a study of 224 chronic pain patients receiving 'commonly-used' doses of oral opioids, in more typical clinical scenarios, found that the opioid-treated patients actually experienced no difference in pain sensitivity when compared to patients on non-opioid treatments. The authors conclude that opioid-induced hyperalgesia may not be an issue of any significance for normal, medically-treated chronic pain patients at all.

Opioid-induced hyperalgesia has also been criticized as overdiagnosed among chronic pain patients, due to poor differential practice in distinguishing it from the much more common phenomenon of opioid tolerance. The misdiagnosis of common opioid tolerance (OT) as opioid-induced hyperalgesia (OIH) can be problematic as the clinical actions suggested by each condition can be contrary to each other. Patients misdiagnosed with OIH may have their opioid dose mistakenly decreased (in the attempt to counter OIH) at times when it is actually appropriate for their dose to be increased or rotated (as a counter to opioid tolerance).

The suggestion that chronic pain patients who are diagnosed as experiencing opioid-induced hyperalgesia ought to be completely withdrawn from opioid therapy has also been met with criticism. This is not only because of the uncertainties surrounding the diagnosis of OIH in the first place, but because of the viability of rotating the patient between different opioid analgesics over time. Opioid rotation is considered a valid alternative to the reduction or cessation of opioid therapy, and multiple studies demonstrate the rotation of opioids to be a safe and effective protocol.

Many medications and conditions can cause sun sensitivity, including:

- Sulfa used in some drugs, among them some antibiotics, diuretics, COX-2 inhibitors, and diabetes drugs.

- Psoralens, coal tars, photo-active dyes (eosin, acridine orange)

- Musk ambrette, methylcoumarin, lemon oil (may be present in fragrances)

- PABA (found in sunscreens)

- Oxybenzone (UVA and UVB chemical blocker also in sunscreens)

- Salicylanilide (found in industrial cleaners)

- St John's Wort, used to treat clinical depression

- Hexachlorophene (found in some ℞ antibacterial soaps)

- Contact with sap from Giant Hogweed. Common Rue (Ruta graveolens) is another phototoxic plant commonly found in gardens. Phototoxicity caused by plants is called phytophotodermatitis.

- Tetracycline antibiotics (e.g., tetracycline, doxycycline, minocycline)

- Benzoyl peroxide

- Retinoids (e.g., isotretinoin)

- Some NSAIDs (e.g., ibuprofen, naproxen sodium)

- Fluoroquinolone antibiotic: Sparfloxacin in 2% of cases

- Amiodarone, used to treat atrial fibrillation

- Pellagra

Photo dermatitis can also be caused by plants like the Dictamnus (commonly known as the "Burning Bush") which is a genus of the flowering plant in the Rutaceae family. This is called phytophotodermatitis.

Allergic rhinitis is the type of allergy that affects the greatest number of people. In Western countries, between 10 and 30 percent of people are affected in a given year. It is most common between the ages of twenty and forty.