University of Maryland Extension

Air Pollution - Trees

yellow speckled leaf

Key Point

  • Air pollution damage often appears as yellowing or flecking of the leaves and foliage may be thin or stunted. Symptoms associated with air pollution damage will vary according to season, environmental conditions, and amount and length of exposure.

Air pollution damage on broadleaf plants

ozone damage on leaf

Ozone is the most widely spread air pollutant in the US. (See photo left.) Acute symptoms vary from stippling, flecking, bleaching or dead areas. Chronic injury develops more slowly over days or weeks. Upper surfaces of broadleaf plants may have tan, red, brown, purple or black coloration. Symptoms are often more severe on leaves exposed to direct sunlight. Newly expanded leaves are most sensitive. Damage symptoms may fade after several weeks.


Sulfur dioxide symptoms appear as ivory to brown interveinal necrosis (dead tissue between leaf veins). Uninjured tissue next to the veins remains green.

Pan (Peroxyacetyl Nitrate) exposure symptoms include patchy silvering or light tan glazing of lower leaf surfaces. Affected leaf may exhibit spots or patches of papery thin almost transparent tissues.

Nitrous oxide causes yellowing of leaf margins and interveinal chlorosis.

Air pollution damage on needled evergreens 

Ozone is the most damaging air pollutant in Maryland and shows up as mottled yellowing or tip browning on needles. Yellowing usually occurs when plants are exposed to low doses of ozone and tip burn appears from exposure to high doses. This tip browning results from necrotic banding of medium-aged tissue along the middle of needles, which is the most sensitive. Tip burn symptoms affect all of the needles equally on a branch. These dead needle tips may also break off over time giving the appearance of shorter than normal needles.

yellowing of pine needles
Mottling and yellowing of needles on pine

air pollution on mugo pine
Air pollution damage on mugo pine

Sulfur dioxide injury on young needles appears as reddish-brown needle tips, but later in the season may appear as bands of necrotic tissue on the needles. Injured needles may remain on the plant for some time.

Fluoride injury generally appears as tip burn with a distinct margin between living and dead tissue.

Rev. 2020

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