Claimant, a printing pressman, suffered a disabling attack of bronchial asthma as a result of his exposure to shop dust and other conditions in the printing plant of the employer appellant. It is undisputed that the claimant suffered from a pre-existing allergy of long standing. He had sustained a series of disabilities as the result of exposure during his prior employment in other printing plants. In each of the prior cases the disability had been charged against the employer in whose plant the exposure had occurred and the case had been closed. Appellants contend that each prior exposure and consequent disability had a cumulative effect on the underlying asthmatic sensitivity from which claimant suffered, and accordingly that there should be an apportionment among all claimant's employers, responsible for such exposure, of the award granted for the latest disability. There was a conflict in the medical opinion as to whether claimant's underlying sensitivity remained constant or was increased as a result of each individual exposure. The board found that each prior individual exposure had caused a temporary aggravation of the pre-existing allergic condition but it found, in effect, that the underlying sensitivity of the claimant remained constant, notwithstanding his many exposures. This finding was supported by substantial evidence. Decision and award unanimously affirmed, with one bill of costs to be divided among the various employer and insurance carrier respondents.