Tropospheric ozone (O₃) levels are predicted to stay high, being a factor within "global change" with potential effects on the carbon sink strength of forest trees. Hence, new approaches to O₃ risk assessment and their validation are required, although appropriate databases for adult trees are scant. Approaches based on external O₃ exposure are presently being evaluated against the ones on O₃ flux into leaves, as the cumulative uptake has the capacity for deriving O₃ risk from cause-effect relationships. The effective dose, however, needs to account for the trees' O₃ defence and tolerance in addition to O₃ uptake. The current status of promoting the preferable mechanistic O₃ flux concept is highlighted for major regions of Europe, addressing refinements and simplifications needed for routine use. At the pan-European scale, however, the flux-based concept is ready for use in O₃ risk assessment and has the potential of meso-scale application at the forest ecosystem level.