(1) persons with self-reported CI sensitize to olfactory and chemosomatosensory stimuli, whereas non-intolerant individuals habituate; (2) sensitization in CI is similar in terms of brain activation patterns to both non-clinical sensitization and other unexplained illnesses such as fibromyalgia; (3) persons with CI have an attention bias to chemical exposures, reflected by problems with withdrawing attention from such stimuli; (4) measures of peripheral hyperreactivity are correlated with chemosensory ERP measures; but failed to corroborate (5) the reactions of women resemble those found in persons with CI to a greater degree than the case in men.Three major theories of CI are also discussed. The neural sensitization theory describes CI as pathological and non-immunological increases in neural responsiveness. The conditioning theory describes CI as the result of basic associative learning mechanisms.