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 Devon plastics factory which resulted in hazardous chemicals released into the surrounding environment.
During the first six hours, many of the local residents were evacuated. But it was then decided that remaining residents should stay in their homes. Researchers looked at the number of 'cases' in both groups - i.e. 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 1096 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).
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 - they compared how many of these symptoms were typically suffered by a neighbouring community unaffected by the fire, with those in the affected town. With an average of 0.48 symptoms per person in the nearby town, the researchers defined a case in the affected town as someone with at least 4 symptoms.
The authors stress that there were several limitations with this study, including a lack of data on whether the level and nature of the smoke exposure could have been different between the groups.
Sheltering may have been a better protective action than evacuation in this chemical incident, they conclude, which confirms existing expert advice recommending sheltering in serious chemical air pollution incidents.
An accompanying commentary agrees that existing emergency planning advice also advocates sheltering during chemical air pollution incidents for those living nearby hazardous sites, at least for the time of peak exposure to dangerous chemicals.
But the stay indoors strategy may not always be appropriate. 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.