Cabin Ozone: A Potentially Serious 'Poison' At High Altitude
As pilots, among our foremost duties is the protection of the safety and health of our passengers as well as ourselves. And yet, there is a potentially serious “poison” at high altitude that can cause long-term health effects on all aboard and about which you’ve likely never been trained. It’s ozone.
The advantages of high-altitude flight are clearly evident. Generally, there is less weather or the convective turbulence found at lower altitudes. Moreover, our turbine engines are optimized for high altitudes, so fuel efficiency is better there and the thinner air means less drag so we can fly farther and faster. As a result, many business jet pilots and passengers will spend thousands of hours breathing the air from our aircraft ventilation systems at 40,000+ ft.
One downside of high-altitude cruise, especially above the tropopause, is the presence of significant concentrations of ozone in that rarefied atmosphere. Stratospheric ozone is considered beneficial in protecting the earth from potentially harmful ultraviolet radiation.
However, exposure to the colorless gas, which is formed from oxygen by electrical discharges or ultraviolet light, is known to cause adverse health effects. Persons exposed to ozone can experience headaches; fatigue; shortness of breath; chest pains; nausea; sinus irritation; coughing; irritation of the eyes, nose or throat; asthma-exacerbation; pulmonary distress; and premature mortality in susceptible passengers. Some symptoms attributed to jet lag may actually be caused by ozone.
Ozone can damage through emphysema, an irreversible condition involving loss of elasticity of the structure of the lungs. Chronic exposure has been tied to reduced lung function in young adults and adult-onset asthma in males, as well as a significant increase in the risk of death from respiratory causes.
The low concentration of ozone below the tropopause is not likely to create health problems for aircraft occupants. Thus, cabin ozone levels for relatively short flights or in aircraft that don’t climb above the tropopause are unlikely to approach threatening levels. However, flight above the tropopause puts the aircraft into potentially threatening levels of the gas.
The tropopause forms a definitive demarcation above which the concentration of ozone increases rapidly to potentially harmful amounts. The tropopause height varies with geography and seasons, as well as locally due to significant storms that cause mixing of the air between the stratosphere and troposphere. Common literature on the tropopause says the average height is 49,000-59,000 ft. (15-18 km) in the tropics to 22,000 ft. (6.8 km) near the poles. Advanced research has found the change in tropopause height with latitude is neither linear nor constant but instead exhibits considerable jumps across the jet stream. In spring, the tropopause height is at a seasonal minimum and in the fall it is at a season maximum in the northern hemisphere.
One of the most extensive collections of inflight data on tropopause height and upper atmospheric ozone levels was collected by the MOZAIC (Measurement of OZone and water vapor by Airbus In-service airCraft) project. The aircraft were fitted with sensors on their shells. Researchers from the University of California-Berkeley’s Department of Civil and Environmental Engineering looked at MOZAIC data from Airbus A340 flights between Munich and Los Angeles (175 flights), Chicago (372 flights) and New York (318 fights) that occurred from 2000 to 2005. They used the inflight data to assess ozone levels encountered, to evaluate the influence of season, latitude and altitude on the levels, and to consider implications for exposures within aircraft cabins.
They discovered that the ozone levels varied considerably across the 865 flights, illustrating in part that they vary markedly through the year. The highest amount of ozone recorded in 1-hr. time periods throughout the flights ranged from 90 to 900 parts per billion (0.09 ppm to 0.9 ppm), while the flight-average atmospheric ozone level was 50 to 500 ppb (0.05 to 0.5 ppm). That is a tenfold variation. We’ll briefly touch on why such variation complicates ozone-avoidance strategies.
UC Profs. Seema Bhangar and William Nazaroff found the average and 1-hr. peak levels were, respectively, 180 and 360 ppb higher in April than during October-November. Why? Flights at normal cruising altitudes of large transports have a higher chance of crossing into the lower stratosphere and encountering elevated ozone in the spring than during the fall.
Since the tropopause is lower over the polar regions, one might think that the flights from Munich to Los Angeles, which traverse a more northerly route, would encounter higher average ozone concentrations than those to Chicago and New York. Surprisingly, that wasn’t the case. Bhangar and Nazaroff did not find a systematic increase with latitude in the ozone concentrations encountered by transatlantic flights between 30 deg. north and 60 deg. north.
Quite the contrary, the data showed that flights to Chicago and New York flying on northern midlatitude routes routinely experienced higher average ozone concentrations because of a high ozone region centered in the western North Atlantic. By traversing higher latitudes, flights between Munich and Los Angeles avoid much of the “high ozone” of that region. Another important finding from this inflight data was that the highest atmospheric ozone concentration levels occurred during occasional localized reductions in tropopause height in January-March in that western North Atlantic region in complex upper atmospheric mixing processes associated with deep storms.
Flight route planning is one method that aircraft operators have used to comply with ozone FARs. Data to aid with flight planning are based on statistical summaries of atmospheric ozone as a function of altitude, latitude and month. However, Bhangar’s inflight data suggest that within the transatlantic flight corridor, latitude is not associated with a smooth linear increase in the tropopause’s height. So, flight route planning based on expected latitude trends may not be effective. The statistical tables are based on averages that do not completely capture the considerable variation of atmospheric ozone concentrations.
How much ozone might be in the cabin air of your aircraft? It depends. The amount of ozone inside depends not only on the atmosphere outside, but also on the aircraft’s ventilation system, materials making up the cabin surfaces, density of occupants and ratio of surface area to volume. (The latter three topics get into areas of organo-chemistry that delve into discussions not likely to be of interest to 99.9% of pilots unless they have an advanced degree in the science.) The rest of this article will focus on aspects that pilots can control.
Researchers from the Harvard School of Public Health monitored ozone concentrations in passenger cabins of 106 commercial flights on domestic, Pacific and Southeast Asian routes. One-fifth of the measurements exceeded 100 ppb. According to both the FARs and JARs, when above 27,000 ft. for each flight segment that exceeds 4 hr., ozone concentrations must not exceed an average 100 ppb. So, the data suggest that those aircraft either lacked converters or their converters were not functioning effectively. And slightly more than one in 10 of the flights exceeded 120 ppb, the U.S. Environmental Protection Agency’s short-term national Ambient Air Quality Standard for ozone. Seasonal comparison showed that cabin ozone levels were higher during the winter and spring than for the summer and fall, which would be consistent with the seasonal variation in the height of the tropopause. Cabin ozone concentrations on the northern Pacific routes were higher than concentrations for other Pacific flights.
A team from UC-Berkeley, again involving Bhangar and Nazaroff and others, also monitored cabin ozone levels on 76 commercial passenger flights on domestic U.S., transatlantic and transpacific routes during 2006 and 2007. On four (out of 68) domestic flights, ozone levels exceeded federal limits of 100 ppb even though 22 of 68 aircraft sampled were equipped with ozone catalysts. The “mean peak-hour” ozone level was one-tenth (only 4.7 ppb by volume) compared with the 46 airplanes not equipped with catalysts (47 ppbv). All eight aircraft sampled during transoceanic flight segments were equipped with ozone catalysts and showed ozone levels well within FAA limits. The researchers also found that the flights with the highest levels of ozone coincided with winter-spring storms that are linked to complex upper atmosphere mixing processes between the lower stratosphere and the upper troposphere.
The UC-Berkeley team estimated that more than 95% of the flights between February and June analyzed in the study would have exceeded the 100 ppb (0.1 ppm) mark for flight-average ozone levels inside the cabin if ozone converters were absent or ineffective. The Berkeley team expressed concern that even on domestic U.S. routes — which are frequently traversed by airliners unequipped with ozone converters — elevated ozone levels of hundreds of ppb are routinely encountered in the winter and spring months.
According to the National Research Council (NRC) committee on air cabin quality’s special study, “The Airliner Cabin Environment and the Health of Passengers and Crew,” “It appears that an ozone converter on large transport category airplanes may be the most robust methodology to ensure consistent, successful compliance with regulations governing airplane ozone control.” The NRC study also highlighted that flight attendants are more likely to be adversely affected by cabin ozone as the level of discomfort is proportional to the level of physical activity.
Ozone is chemically unstable and its decomposition is accelerated by heat and contact with metallic surfaces. But even though bleed air extracted from the engine’s compressor is plenty hot and the piping is made from metal, those factors may not be sufficient to mitigate the ozone threat when outside concentrations are elevated. Thus, to meet the limitations on cabin air ozone levels, high-flying transport aircraft should be equipped with fully functional catalytic devices and carbon filters that remove gas from the cabin.
Catalytic ozone converters typically consist of a metal housing that encloses a precious-metal catalyst. The cores are coated with chemical compounds that, when combined with the elevated temperatures of the bleed airflow, become the catalyst in converting ozone to standard oxygen. The converters have an expected ozone destruction efficiency of 90%-98% when new. However, that efficiency tends to degrade with use.
According to RSA Engineered Products, an engineering company that has designed a dual core ozone converter, other converters do not always perform well due to surface “poisoning” by various contaminants or imperfect refurbishing of catalysts during scheduled replacement. The accumulation of particles on their absorbent surface further decreases the efficiency of carbon absorption filters. This leads to costly filter maintenance or replacement, and until then, the converter’s effectiveness is likely to allow excessive concentrations of ozone into the cabin.
Ozone converters are subject to replacement or maintenance once the efficacy drops below approximately 60%. At that level, the UC-Berkeley study predicted that 97% of flights from February through June would exceed the peak 1-hr. ozone level of 100 ppb. These observations highlight the importance of ozone converters functioning well.
How can a pilot detect the presence of ozone? Unfortunately, the human nose is not well suited for the job. Even when ozone is detected by smell — it can be pungent — this sense diminishes after a few minutes, giving the illusion that the gas’s level has fallen even though it has not.
According to the Australian Civil Aviation Safety Authority’s study, “Contamination of Aircraft Cabin Air by Bleed Air: A Review of the Evidence,” there is clear evidence of increased levels of a range of potentially hazardous contaminants during routine operating conditions, including ozone when flying at high altitude.
The NRC report expressed concern that “In addition, future airplanes will be able to cruise at higher altitudes in the stratosphere where the concentration of external ozone is much higher than in the troposphere.” That report focused on large transport aircraft, whereas many business jets had been capable of flying at high altitudes well above the tropopause long before its publication. The safety implication is obvious as business aircraft commonly fly much higher than large commercial transports and thus through atmospheric regions laden with high concentrations of ozone.
High-altitude flight involves numerous unseen hazards, including ozone concentrations. Ignorance of its threat highlights one of the deficiencies in pilot training for high-altitude flight. Unfortunately, the studies on this topic have concentrated on the air quality within airline cabins. The protection of pilots, flight attendants and passengers, particularly in high-altitude business aircraft, deserves equal attention.