For broadleaf species, visible foliar ozone injury can be identified and distinguished from symptoms caused by other biotic/abiotic factors:

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- Visible ozone injury occurs on fully developed leaves that are exposed to full sunlight.

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- Symptoms are more severe on mid-aged and older leaves than on younger leaves. Older leaves are the first ones to develop symptoms (age effect).

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- Shaded portions of the leaves (i.e. if two leaves overlap) usually do not show any injury (shade effect).

- Visible ozone injury normally does not go through the leaf tissue. Visible symptoms are most likely confined to the upper leaf surface, typically expressed as tiny purpled, yellow or black spots (described as stipples) or sometimes as a general even discoloration, reddening or bronzing.

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- Both stippling and even discoloration only occur between the veins (interveinal) and do not affect the veins.

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- Towards the end of the growing season, foliar symptoms may progress to leaf yellowing or premature senescence. 

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