At first, one experiment was made for research on the relationship to eye irritation by
irritants in synthetic oxidant from 1975 to 1976. Examined irritants were formaldehyde,
peroxy acetyl nitrate (PAN), peroxy benzyl nitrate (PBzN). Thresholds and grades of eye
irritation were studied with each of a single irritant gas and compound irritant gases, because
an air pollution was composed of various irritants.