Scientists in the US have used data from sensors on aircraft to show that current strategies for reducing passenger exposure to ozone may not be effective.

Although ozone high up in the stratosphere provides protection from ultraviolet radiation, it can be harmful when circumstances such as air travel or ground-level pollution expose humans to high levels of the gas. Ozone can cause breathing difficulties, headache or even premature death. As a result, US Federal Aviation Regulations limit ozone levels to less than 0.25 parts per million, and to 0.1 parts per million as a three-hour average. Now Seema Bhangar and Bill Nazaroff of the University of California Berkeley are arguing that the route-planning measures used to limit ozone exposure may not be up to the job.

“[Our research] raises questions about the claim – which is written into current federal regulations – that aircraft can prevent high in-cabin ozone levels by modifying flight paths to avoid air that is expected to contain high concentrations of ozone based on statistical summary tables of the variation of ozone with altitude, latitude and season,” explains Bhangar. “The summary tables do not capture the rich spatial variation of atmospheric ozone concentrations.”

The team found that the mean height of the tropopause – the boundary between the troposphere and stratosphere – decreased towards the poles, as expected. This should mean that ozone concentrations at aircraft cruising altitudes are generally larger at high latitudes. But the pair did not find a systematic or monotonic increase with latitude in the ozone concentrations encountered by transatlantic flights.

Local disturbances

“Our research findings suggest that the highest ozone levels encountered by aircraft may be tied to local, episodic disturbances in the tropopause height,” says Bhangar. “These high levels cannot be avoided through flight-path planning that is based only on mean annual variations in ozone with altitude, latitude and season.”

Many aircraft use ozone control devices, or “converters”, in their ventilation systems to remove ozone entering the plane. The team’s results indicate that as these devices age and approach the 60% efficiency limit at which they receive maintenance, cabin ozone levels between February and June could exceed the 0.1 ppm limit on transatlantic flights; the tropopause tends to be lowest in the spring and at its maximum height in the autumn.

“Though levels of ozone outside the cabin do not translate directly to exposures inside the cabin, outside ozone concentration is a key indicator of in-cabin exposures to both ozone and ozone-reaction byproducts,” says Bhangar. The presence of an ozone control device is also important.

A domestic issue

Currently, only long-haul flights are required to have ozone control devices. But the team found that ozone levels greater than 0.1 ppm were routinely encountered outside the aircraft even in US domestic airspace.

In earlier studies Bhangar, Nazaroff and colleagues took a suitcase full of ozone measurement kit on board planes. Arranging this is complicated, expensive and time consuming, so when the researchers heard about the vast amount of data collected by the MOZAIC project, they were intrigued. MOZAIC has been running since 1993, using sensors installed on the shells of commercial aircraft.

The researchers looked at MOZAIC data from Airbus A340 planes travelling from Germany, to either Los Angeles, Chicago or New York between 2000 and 2005 – a total of 865 flights.

Opportunistic and valuable

“This approach – making opportunistic use of an existing dataset – started as an engaging diversion from our primary air-sampling-based investigation, but proved to be a valuable complement to that campaign, which provided us with independent, useful insights,” says Bhangar. “The present work provides a way to build on the in-cabin work via a much larger sample size and lower investment of sampling effort per monitored segment.”

Peak one-hour ozone concentrations measured in the study ranged from 90–900 ppb, while the flight-average ozone level was 50–500 ppb. Flights to Chicago and New York typically saw higher average ozone concentrations than those to Los Angeles, as flights to the US West Coast tend to take a more northerly route, avoiding much of the “high ozone” region centred in the western North Atlantic, say the researchers.

The amount of ozone inside the cabin depends not only on the atmosphere outside, but also on the ventilation system, materials making up the cabin surface, density of occupants, and surface area to volume ratio. The team estimated that more than 95% of the flights between February and June analysed 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.

Reducing human exposure

“Our results improve the scientific basis for understanding conditions under which air travellers and flight crew are likely to be exposed to elevated ozone,” says Bhangar. “The findings can help regulators and the airline industry to be more efficient, effective and strategic in reducing human exposures to ozone and ozone byproducts in the air-cabin environment.”

The ideal next step, Bhangar believes, would be to add an indoor monitoring component to the MOZAIC endeavour. “If even one of the MOZAIC aircraft currently measuring outdoor parameters on all routine flights were equipped with an indoor ozone sensor, the resulting data – in conjunction with information on the ventilation system, ozone converter and occupant load – could give us an unprecedented look not only at real-time in-cabin and atmospheric ozone levels on a large number of flights, but also at what mediates the relationship between these two variables.”

In the meantime, the scientists are investigating levels of dynamic air pollutants in other occupied indoor environments, with a focus on bioaerosols. They reported their aircraft results in Environmental Research Letters (ERL).

Ozone is not the only health issue facing air travellers. Levels of neutron radiation on board aircraft are also monitored and some physicists are concerned about cosmic rays.