Blackout gave cities a breath of fresh air

The blackout that left 50 million North Americans without power in August 2003 had an unexpected benefit - the air became cleaner.

His team from the University of Maryland in College Park flew an aircraft over the middle of the blackout zone 24 hours after the power had gone down. "This was a unique opportunity to explore what would happen to air quality if power station emissions were reduced," he says.

Sulphur dioxide levels decreased by 90 per cent, there was around half the amount of ozone and visibility increased by 40 kilometres.

Measurements from the US Environmental Protection Agency show that during the blackout, emissions from affected plants dropped by two-thirds or more. However, measuring the effect of this on air quality was difficult because many ground-based stations were also blacked out, being powered by electricity.