| Oxidants such as ozone, oxides of nitrogen and
peroxyacetyl nitrate (PAN) cause the most direct
damage to plants. These are followed by sulfur
dioxide. Fluorides, chlorides, ammonia, and chlorine
cause occasional local damage. Metallic elements
from polluted air also accumulate in plants, but their
effects at the concentrations encountered in most
localities are minor or unknown.
Injury from air pollutants can be put into two major
categories: (1) chronic injury, which is the
cumulative effect of long exposure (often over
several seasons) to toxic pollutants at low levels on
sensitive species or at moderate to high levels on
tolerant species; (2) acute injury occurs after short
exposure (a few hours to several days) to pollutants
at high levels on tolerant species or at moderate to
low levels on sensitive species. Acute injury
commonly occurs following accidental leaks or
spills of gas or volatile liquids. Acute injury results in
moderate to severe foliar injury and is the most
frequently diagnosed type of air pollution injury.
Chronic injury is difficult to diagnose because the
foliar symptoms are mild or nonexistent and the
trees decline over several years.
Most air pollution results from electric power
generation, cars and trucks, and a variety of
industrial operations. Air pollutants that are toxic to
trees can occur in several forms: invisible gasses,
particulates (smoke or dust), or aerosols (fine mists).
Invisible gasses are the most common form of
pollution toxic to plants (phytotoxic) rather than
smoke. Air pollution is not restricted to urban and
suburban areas but is also found in remote areas.
Symptoms and Diagnosis
Diagnosis of air pollution damage is difficult.
Symptoms can be very similar to those produced by
stress from moisture, temperature, or nutrient
disorders as well as a variety of biotic factors such
as bacteria, fungi, viruses, sucking insects, and
mites. Therefore, once these factors are eliminated,
one can look to air pollution as the source of the
problem. Local meteorological conditions and
nearby sources of emissions can often be
misleading. The source, the specific sensitivity as
correlated with symptom type, and the nature and
the movement of air pollutants must be known
before accurate diagnosis of injury can be achieved.
Sensitive chemical analyses are sometimes needed
to detect air pollutants.
Because of the complexity of symptoms, it is
suggested that reference material be used to further
investigate symptoms.
See individual sheets on Ozone and Sulfur Dioxide
for how to diagnose these two problems.
Life Cycle
Specific effects of air pollutants on plant tissue vary
with the pollutant, host, time of year, and numerous
meteorological factors such as temperature, relative
humidity, wind, and solar radiation.
Ozone builds up to phytotoxic levels in the
atmosphere during warm, sunny weather when
pollutants accumulate in stagnant air. Accumulation
often occurs during atmospheric inversions where
warm air is trapped in valleys and basins bounded
by mountains by a layer of cool air above.
Juvenile leaf tissues are most susceptible to acute
injury of fluorides and consequently most severe
injury occurs in the spring. Chronic injury, however,
occurs progressively during the entire growing season.
Integrated Pest Management Strategies
1. Maintain plant vigor. Plants in good health resist
all types of injury better than weakened plants.
Therefore, water in times of drought and fertilize
to maintain nutrient balance. Soil should be
porous, well-draining, and not compacted.
2. Reduce emissions. Reduce emissions from the
source when possible. | 
