Pollutants rapidly become diluted and blown away by upper level winds before contacting many trees. These inversions usually occur in narrow valleys close to mountain ranges, and near large bodies of water. Temperature also influences a pollutant's chemical reaction rate. When the temperature is high, more photochemical oxidants are produced. These oxidants cause more severe damage during sunny hot weather and less damage during cool cloudy weather.
Atmospheric moisture causes pollutants to become solutions, increasing their toxic potential.
Chemical and physical nature of the pollutant. Geographic and topographic influences. Length of exposure and concentration. Types of symptoms. Possible mimicking symptoms.