Preservation of products with the combination of 5-chloro-2-methyl-4-isothiazolinone (methylchloroisothiazolinone, MCI) and 2-methyl-4-isothiazolinone (methylisothiazolinone, MI), commercialized as Kathon CG or WT, is widely used. Allergic contact dermatitis reactions to the preservative have been described in many publications of epidemiological studies. In addition, a number of allergic, airborne reactions to MCI/MI were observed in recent years. The aim of the presented study was: (i) to compare epidemiological data of MCI/MI with data of other preservatives; (ii) to estimate the possible exposure to MCI/MI from commercial products and the distribution of MCI/MI in chemical products based on a product evaluation of the chemical product register of the Swiss Federal Office of Public Health; (iii) to estimate the exposure to MCI/MI, volatile organic chemicals (VOC) and glycols from freshly painted walls; (iv) to compare the contact allergenic potential of MCI/MI with other preservatives. It was found that the number of registered chemical products, unambiguously identifiable as containing MCI increased from 3644 products in July 1999 to 4843 products in April 2001 in Switzerland. It is estimated that today, the total number of chemical products preserved with MCI/MI on the Swiss market is about 6000. A further increase is to be expected. The contact allergenic potential of MCI/MI seems to be higher than for other preservatives. Measurements of MCI/MI concentrations in air samples of freshly painted rooms have shown that the concentration of MCI/MI in the air a few days after painting may be high enough to elicit airborne reactions in already-sensitized patients. In a single case, reactions to MCI/MI were observed over months after wall painting. In this case, both wall paint and plaster contained MCI (40 and 46 ppm). Volatile organic chemicals (VOC) may also be relevant for the elicitation of airborne reactions. In 1/3 of the measurements, the VOC concentrations measured within 14 days after painting were in a range within which sensory irritation and discomfort may be induced. Declaration of MCI/MI-containing products, replacement of MCI/MI by alternative preservatives or the separate use of preservatives in industrial products and in cosmetics might reduce the risk of airborne reactions.