There has not been extensive research into environmental causes of hyperosmia, but there are some theories of some possible causes.

In a study by Atianjoh et al., it has been found that amphetamines decrease levels of dopamine in the olfactory bulbs of rodents. On this basis, it has been hypothesized that amphetamine use may cause hyperosmia in rodents and humans, but further research is still needed. Anecdotal support for the belief that amphetamines may cause hyperosmia comes from Oliver Sacks's account of a patient with a heightened sense of smell after taking amphetamines.

It has been observed that the inhalation of hydrocarbons can cause hyperosmia, most likely due to the destruction of dopaminergic neurons in the olfactory bulb.

Methotrexate, administered in the treatment of psoriasis, has been known to cause hyperosmia, and may be more likely to do so in patients with a history of migraines. However, this is only an observation and not part of a study, therefore it is yet to be verified.

A longitudinal study on pregnant females found that 76% of pregnant females experienced significant changes in gustation and olfaction perception. This was found to be caused and linked to their pregnancy. The study concluded that 67% of the pregnant females had reported a higher level of sensitivity to smell, 17% suffered from an olfactory distortion and 14% suffered from phantosmia; these distortions were very minimal towards the last stages of pregnancy and in the majority were not present post partum. Furthermore, 26% of these participants also claimed that they also experienced an increased sensitivity to foods that were bitter and a decreased sensitivity to salt. These findings suggest that pregnant females experience distorted smell and taste perception during pregnancy. It has also been found that 75% of women alter their diets during pregnancy. Further research is being conducted to determine the mechanism behind food cravings during pregnancy.

A study by Menashe et al. has found that individuals with a single nucleotide polymorphism variant in the OR11H7P pseudogene have a lower receptor activation threshold for isovaleric acid. These individuals are hyperosmic for this single odorant.

Another study by Keller et al. has found that people with the intact human odorant receptor OR7D4 are more sensitive to androstenone and androstadienone and thus find them unpleasant (individuals with the semi-functional OR7D4 have two non-synonymous single nucleotide polymorphisms in the OR7D4 pseudogene, resulting in two amino acid substitutions). There has not yet been extensive research into the genetic background of those with general hyperosmia, rather than for just a single odorant.

Phantosmia is most likely to occur in women between the ages of 15 and 30 years. The time of the first hallucination(s) lasts from anywhere from five to twenty minutes. It has also been found that the second hallucination will occur approximately a month later in the same manner as the first. Over time, the length of the hallucination will begin to increase.

A temporary loss of smell can be caused by a blocked nose or infection. In contrast, a permanent loss of smell may be caused by death of olfactory receptor neurons in the nose or by brain injury in which there is damage to the olfactory nerve or damage to brain areas that process smell (see olfactory system). The lack of the sense of smell at birth, usually due to genetic factors, is referred to as "congenital anosmia." Family members of the patient suffering from congenital anosmia are often found with similar histories; this suggests that the anosmia may follow an autosomal dominant pattern. Anosmia may very occasionally be an early sign of a degenerative brain disease such as Parkinson's disease and Alzheimer's disease.

Another specific cause of permanent loss could be from damage to olfactory receptor neurons because of use of certain types of nasal spray; i.e., those that cause vasoconstriction of the nasal microcirculation. To avoid such damage and the subsequent risk of loss of smell, vasoconstricting nasal sprays should be used only when absolutely necessary and then for only a short amount of time. Non-vasoconstricting sprays, such as those used to treat allergy-related congestion, are safe to use for prescribed periods of time. Anosmia can also be caused by nasal polyps. These polyps are found in people with allergies, histories of sinusitis & family history. Individuals with cystic fibrosis often develop nasal polyps.

Amiodarone is a drug used in the treatment of arrhythmias of the heart. A clinical study performed demonstrated that the use of this drug induced anosmia in some patients. Although rare, there was a case in which a 66-year-old male was treated with Amiodarone for ventricular tachycardia. After the use of the drug he began experiencing olfactory disturbance, however after decreasing the dosage of Amiodarone, the severity of the anosmia decreased accordingly hence correlating the use of Amiodarone to the development of anosmia.

Anosmia can have a number of harmful effects. Patients with sudden onset anosmia may find food less appetizing, though congenital anosmics rarely complain about this, and none report a loss in weight. Loss of smell can also be dangerous because it hinders the detection of gas leaks, fire, and spoiled food. The common view of anosmia as trivial can make it more difficult for a patient to receive the same types of medical aid as someone who has lost other senses, such as hearing or sight.

Losing an established and sentimental smell memory (e.g. the smell of grass, of the grandparents' attic, of a particular book, of loved ones, or of oneself) has been known to cause feelings of depression.

Loss of olfaction may lead to the loss of libido, though this usually does not apply to congenital anosmics.

Often people who have congenital anosmia report that they pretended to be able to smell as children because they thought that smelling was something that older/mature people could do, or did not understand the concept of smelling but did not want to appear different from others. When children get older, they often realize and report to their parents that they do not actually possess a sense of smell, often to the surprise of their parents.

A study done on patients suffering from anosmia found that when testing both nostrils, there was no anosmia revealed; however, when testing each nostril individually, tests showed that the sense of smell was usually affected in only one of the nostrils as opposed to both. This demonstrated that unilateral anosmia is not uncommon in anosmia patients.

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.

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