People who have rushed out after September 11th to buy a gas mask in the hope it will save them from chemical or biological attack have probably bought a gadget that is useless or even dangerous, experts say.
Tens of thousands of gas masks have flown off the shelves since last month's attacks, spurred by fears that terrorists could use toxic gas or a germ weapon in the next assault.
But in many cases, the masks are useless and a lack of training could in fact make them fatal, New Scientist reports in next Saturday's issue.
Masks have a canister containing a carbon filter that absorbs chemicals from the air.
The filter is covered by a protective plastic seal which is removed before the mask is used.
"These filters have a usual lifetime that can be measured in a matter of hours," according to Dr Brian Davey, head of health and safety at the Organisation for the Prohibition of Chemical Weapons in The Hague.
People who have bought a canister which does not have a seal have no way of knowing how much life is left in the filter, he told the journal.
A user also needs proper training to know how and when to use the mask.
More than a dozen Israelis died during the Gulf War because of accidents with gas masks, half of whom suffocated because they failed to remove the seal before putting the mask on.
"They thought they were dying of nerve gas," said Mr Jack Sawicki, an expert in protective clothing at Geomet Technologies in Germantown, Maryland.
"But they just hadn't taken the plug out."
Mask users also have to be clean-shaven to ensure that the mask has an effective seal around the face, to prevent germs or gas fumes from seeping in.
Men with beards or even a day's stubble are likely to compromise the seal.
Even if the mask is in good condition and the owner knows how to use it, the device still gives no protection against the many chemical weapons that can penetrate the skin, and for which a full rubberised suit is needed.
And protective gear in itself cannot help if the user is not alerted in time.
"An attack is unlikely to materialise as a plane going over and a huge cloud coming out of it. It'll arise as sick people," Dr Davey said. "The problem is that people don't have an early-warning system. You'd have to wear a mask all the time."