Spores from mould growing in New Orleans homes flooded after Hurricane Katrina pose a major risk to health, a US environmental group has warned.
New Orleans residents have been warned to wear protective gear
Air tests have shown levels of mould high enough to trigger serious allergic reactions in some people, the Natural Resources Defense Council (NRDC) says.
It accuses the government of doing too little to warn residents of the risks.
But the US Centers for Disease Control and Prevention (CDC) told the BBC that advice had been widely distributed.
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.
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."
DEALING WITH MOULD
Use a N95 standard respirator
Wear nitrile gloves and a head-to-toe body suit
Discard protective clothing after each use
Wash well on leaving the contaminated area
Throw away any non-cleanable items that were wet for over 48 hours or have visible mould
Scrub metal, hard plastics and ceramics with a bleach solution
Reduce moisture by opening windows and running fans
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.
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."