In the first study of its kind, researchers compared the effects of both sheltering and evacuation on the local population during a fire at a plastics factory that resulted in hazardous chemicals released into the surrounding environment.

During the first six hours, many of the local residents were evacuated. But it then was decided that remaining residents should stay in their homes. Researchers looked at the number of "cases" in both groups -- that is, those experiencing symptoms related to the incident.

They found that soon after the fire, the evacuated group had almost twice as many cases as compared with those in the sheltered group (19.7% compared to 9.5%), though the difference did not seem to persist after two weeks (3.3% compared to 1.9%).

The study was based on a health survey involving 1,096 residents in the town -- 797 of whom were sheltered and 299 evacuated. Both evacuated and sheltered sets of residents lived similar average distances from the factory (565m and 572m respectively).

Respiratory Symptoms

The effects of severe chemical air pollution are similar to those of respiratory illness, say the authors -- cough, for instance, or runny eyes.

To distinguish cases -- those whose symptoms were incident-related -- the researchers compared the number of respiratory symptoms typically suffered by a neighboring community unaffected by the fire with the number of symptoms reported in the affected town.

The researchers defined a case in the affected town as someone with at least 4 symptoms. The nearby town reported an average of 0.48 symptoms per person.

There were several limitations with this study, the authors stress, including a lack of data on whether the level and nature of the smoke exposure could have been different between the groups.

In this chemical incident, sheltering may have been a better protective action -- a conclusion that confirms existing expert advice recommending sheltering in serious chemical air pollution incidents.

Peak Exposure Time

While sheltering at home may be advisable during some air-pollution incidents -- at least for the time of peak exposure to dangerous chemicals -- the stay-indoors strategy may not always be best.

Other factors -- such as weather conditions, how the fire is being managed by emergency services, or the type of fire itself -- may make evacuation more appropriate.