of an odorant. selectivity refers to the insects' ability to tell one odorant apart from another. these compounds are commonly broken into three classes: short chain carboxylic acids, aldehydes and low molecular weight nitrogenous compounds. some insects, such as the moth deilephila elpenor, use smell as a means to find food sources. in plants the tendrils of plants are especially sensitive to airborne volatile organic compounds. parasites such as dodder make use of this in locating their preferred hosts and locking on to them. the emission of volatile compounds is detected when foliage is browsed by animals. threatened plants are then able to take defensive chemical measures, such as moving tannin compounds to their foliage. machine-based smelling scientists have devised methods for quantifying the intensity of odors, in particular for the purpose of analyzing unpleasant or objectionable odors released by an industrial source into a community. since the 1800s industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions among nearby residents regarding airborne odor. the basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard. since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air. a field olfactometer can be utilized to determine the magnitude of an odor. many air management districts in the us have numerical standards of acceptability for the intensity of odor that is allowed to cross into a residential property. for example, the bay area air quality management district has applied its standard in regulating numerous industries, landfills, and sewage treatment plants. example applications this district has engaged are the san mateo, california, wastewater treatment plant; the shoreline amphitheatre in mountain view, california; and the it corporation waste ponds, martinez, california. classification systems of classifying odors include: crocker-henderson system, which rates smells on a 0-8 scale for each of four "primary" smells: fragrant, acid, burnt, and caprylic. henning's prism zwaardemaker smell system (invented by hendrik zwaardemaker) disorders specific terms are used to describe disorders associated with smelling: anosmia – inability to smell hyperosmia – an abnormally acute sense of smell hyposmia – decreased ability to smell presbyosmia – the natural decline in the sense of smell in old age dysosmia – distortion in the sense of smell parosmia – distortion in the perception of an odor phantosmia – distortion in the absence of an odor, "hallucinated smell" heterosmia – inability to distinguish odors olfactory reference syndrome – psychological disorder that causes the patient to imagine he or she has strong body odor osmophobia – aversion or psychological hypersensitivity to odors viruses can also infect the olfactory epithelium leading to a loss of the sense of olfaction. about 50% of patients with sars-cov-2 (causing covid-19) experience some type of disorder associated with their sense of smell, including anosmia and parosmia. sars-cov-1, mers-cov and even the flu (influenza virus) can also disrupt olfaction.