The steamy climate in New Orleans and the fact that many homes were under water for days or weeks following Katrina has made it a fertile breeding ground for mould.

'Dangerously uninhabitable'

The authorities have encouraged returning residents and workers to wear masks and protective clothing when cleaning up buildings.

But the NRDC, a national non-profit organisation, has said government agencies must do more to test air quality and equip people with adequate protection.

 

It says lower income households are at particular risk if they try to rebuild their homes without head-to-toe protective suits - which cost about $40 (£23) and should be worn only once - and proper respirators.

Tests it carried out in mid-October at 14 sites across New Orleans, nine of which had been significantly flooded, showed high levels of mould spores both indoors and out.

Dr Gina Solomon, who led the NRDC study, said: "The outdoor mould spore concentrations could easily trigger serious allergic or asthmatic reactions in sensitive people.

"The indoor air quality was even worse, rendering the homes we tested dangerously uninhabitable by any definition."

 

Her team found levels as high as 645,000 spores per cubic metre (35 cubic ft) inside a building in the badly flooded Uptown district and outdoor levels of 81,000.

The US National Allergy Bureau considers outdoor mould counts above 50,000 as "very high".

The work being carried out to clean up and rebuild the city is likely to continue stirring up mould spores for some time to come, Dr Solomon warned.

Those with allergies, lung problems or weak immune systems are at particular risk.

Long-term problem

CDC spokeswoman Jill Smith told the BBC News website it had handed out tens of thousands of leaflets giving "precautionary" advice to New Orleans residents.

"Not all people are the same. Mould affects different people in different ways," she said.

"Certainly asthmatics and young children, pregnant women and the elderly should not be in the areas where mould is present."

However, a study by a health team through October had not found a significant increase in cases of wheezing, pneumonia or asthma attacks, she said.

The problem of mould contamination was likely to continue for months at least, she added.

"This is so unprecedented, this volume of material that we are dealing with. It's hard to estimate how long it's going to take."