Adverse effects of respirators have been the focus of considerable research over the past decade. Individual research projects have generally focused on one specific category of effects: ventilatory, respiratory control, work limitation, subjective discomfort, psychologic effects, thermal loading, and cardiovascular changes. Most were studied in experimental laboratory situations rather than in actual worksites. Very little attention has been given to compliance with use and actual effectiveness in preserving health. Inasmuch as many types of effects have been demonstrated, there is a need to carefully define which type(s) of effects is/are most important for respirator design selection and worker medical certification in particular situations. In general, respirators should be assessed for their effect on all relevant variables.