ATLANTA (Reuters) - US health experts on Thursday advised Americans worried about being exposed to anthrax during the busy holiday mail season not to blow, sniff or peer into opened letters, to avoid jostling or tearing packages and to wash their hands after handling mail.

The Centers for Disease Control and Prevention (CDC) said the recommendations were not scientifically proven, but may reduce the risk of becoming infected with the bacterium that has killed five people and infected 13 others since an outbreak linked to contaminated mail began in October.

"If there is a risk from cross-contaminated mail, it is very low. But because some people are concerned, worried about the situation and want to be able to do something, we've come up with some potential steps that they might take,'' said Dr. Jeffrey Koplan, director of the CDC.

Koplan, who described the US postal system as generally ''very safe,'' said the recommendations were based on common sense and could help make some Americans feel more comfortable about handling the mail.

Koplan's comments came after the CDC released an update on its investigation into the anthrax outbreak, which began just weeks after the Sept. 11 attacks by hijackers on New York and Washington and made an already anxious nation nervous about a possible bioterror threat.

Health experts suspect that the majority, if not all, of the reported anthrax infections were caused by some sort of contact or exposure to contaminated letters sent to media outlets in Florida and New York, as well as Capitol Hill offices in Washington.

Five other cases of anthrax had been suspected in New York and New Jersey, though the CDC said on Thursday it had lowered that number to four after failing to confirm skin anthrax in a 54-year-old New Jersey postal worker.

There has not been a new case of anthrax since Nov. 21 when Ottilie Lundgren, a 94-year-old Connecticut woman, died of inhalation anthrax. CDC officials said on Thursday they still could not explain how Lundgren became infected.

They noted that none of the microscopic spores that cause anthrax had been found in any of the areas Lundgren was known to have frequented, though traces had been detected on sorting machines at a nearby postal facility and on an envelope delivered 4 miles (6.4 km) from Lundgren's home.

"We are continuing to work with the postal service to see of any possibilities of other mail she may have received or what the quality of it is,'' Koplan said.

Despite investigators' failure to trace the transmission route of the anthrax that killed Lundgren and a New York hospital worker who was not an apparent target of the attack but died of the disease in late October, Koplan said the CDC continued to view the risk of getting inhalational anthrax as remote.

"There are billions of pieces of mail that have passed through postal facilities in the last few weeks while this has been going on and we've got a grand total of 18 confirmed and four suspect cases in the course of this,'' said Koplan, who noted that the agency would remain vigilant in looking for new cases.