Urban trees emit biogenic volatile organic compounds (BVOC) that affect air quality through the formation of ozone and particulate matter. Trees in Montreal and Helsinki did not emit more BVOCs than expected based on measurements from forest trees, but the emissions varied between individual trees and growth environments. Avoiding high-BVOC emitting tree species and management strategies that protect trees from BVOC-inducing stress factors would help minimise their negative air quality impacts.