01. What is Ozone and When Can it be Encountered?
Ozone (O₃) is a variable form of the oxygen element and has three atoms in each gaseous molecule, lifegiving oxygen (O₂) has two. Ozone itself is bluish in colour. Ozone is formed naturally by the action of the sun’s short wave-length light on oxygen in the air. It is found high above the earth’s surface in the tropopause, a transition layer between the troposphere and stratosphere. Abnormally high concentrations of Ozone may occur and persist for short periods of time at certain geographic locations and altitudes near and above the tropopause.
The altitude of the tropopause is dependent on various factors, on seasonal influences in particular and on earth latitude. The concentration of Ozone at jet aircraft operating levels is highest during February through April in the northern hemisphere and during August through October in the southern hemisphere. Basically this is because the tropopause is lower at this time of the year, and Ozone tends to descend with the tropopause. During the spring the upper atmospheric winds characteristically transport higher concentrations of Ozone to lower altitudes.
02. How is it Recognized and What is the Consequence?
At low concentration Ozone is recognizable by its odor described by some people as an “electrical” smell. The human sense of smell adapts quickly and therefore odor is not a reliable indicator of the presence of Ozone at concentrations that may cause irritation.
The Ozone concentration is expressed in parts per million. At the higher flight levels the ambient air contains a much higher concentration of Ozone than the cabin air. It was found that roughly 50% of the ambient Ozone concentration enters the cabin through the airconditioning system. Exposure to Ozone concentration below 0.3 ppm for about one hour is usually non-symptomatic. Exposure to concentrations of about 0.3 ppm causes irritation to the eyes, nose, mouth and throat, like shortness of breath, coughing and, in rare cases, chest pains. Also a decrease in night vision ability has been reported after exposure to 0.2 ppm for three hours or more. The degree of irritation generally increases with increasing levels of Ozone concentration exposure time and physical activity.
The irritation does not cause permanent damage to tissues or organs, and should disappear as soon as the aircraft has passed a high concentration area or has descended to a lower level.
03. What Can be Done About it?
As soon as the first signs of Ozone irritation are reported the following might be considered. The flight crew has to request a flight level lower than the actual Ozone level. 4000 ft would be a reasonable first try. If the Ozone region cannot be avoided, pilot on night flights should use 100% oxygen sometime prior to their descent to landing to counter the known harmful effects of Ozone on night vision.
Irritation can be more annoying to persons with bronchial problems or to those who are sensitive to respiratory inflammation. It takes a longer time for these persons to recover from the temporary effects. It is advisable to remain as immobile as possible and to limit the use of alcoholic beverages, since physical activities and the use of alcoholic drinks increase the Ozone irritation.
To relieve from Ozone effects, breathing 100% oxygen or breathing through a warm, moist towel or equivalent held firmly around the nose and mouth is recommended.
Various laboratory tests have shown that a deficiency of Vitamin E results in increased susceptibility to the harmful effects of inhaled Ozone. Likewise, excess Vitamin E appears to afford a limited protective effect.