Airborne exposure to sensory irritants evokes in man a sensation which may increase to a burning and painful sensation in the nasal passage and the cornea due to stimulation of the trigeminal nerve endings. I The unpleasant sensation, termed sensory irritation,'is important in relation to occupational exposure and, thus, the effect constitutes the basis for many occupational threshold limit values (TLV). 2'3 Sensory irritation is also observed in relation to indooP as well as outdoor air pollutants. 7-9 The effect offers a possibility to classify thermal decomposition products of polymers'o." and for evaluation of other strongly irritating substances as anti-riot control agents.'The sensation from the trigeminal system depends, however, very much on the substance and the concentration. The activation at low concentrations may appear as nonpainful tactile stimuli, 12 as a~ arm'~ 9'~ or~ 0 0 1'~ sensation. A somewhat higher degree of stimulation may give rise to irritation sensation^'^ such as piquancy, I2~ tinging,'~.'~ ti~ kling,'~.'~~ ungency,'~.'~ and increase to a painful and burning sensation. 1, 12-16 Sneezing is another effect which can be mediated by the trigeminal system. 15917-19

Different aspects of the trigeminal system related to sensory irritation have previously been reviewed: evolution, 20* 21 types of nerves involved, I. I4, 15, l8, 19, 21-26 reflexes,'.'8.2', 26, 27 structureactivity relationships,', 3.20'21 occupational hygiene consequences, 2, 3, 27 and perceptions. 14, 28 This review will not deal with the original literature already reviewed except where justified for special purposes. The review intends to update the authoritative review'within the area of activation mechanisms of the sensory irritant receptor by airborne chemicals. Differences between species and strains as well as description of methods will only be referred to if relevant for the understanding of the receptor activation mechanisms. Also, an integration of the knowledge developed during the past 15 years is presented with a discussion of models relevant to a further understanding of this area.