Filed at 3:51 p.m. ET

WASHINGTON (AP) -- The head of the federal Centers for Disease Control and Prevention told postal workers Friday that medical experts are providing their best advice on anthrax, but the disease isn't well enough understood to tell people for sure what to do.

``There have been lots of unanswered questions,'' Dr. Jeffrey P. Koplan told a meeting of the American Postal Workers Union.

``We've learned a lot'' since the anthrax-by-mail attacks last fall, he said, including that some of the answers they had before the attacks were wrong.

Decisions have to be made every day, Koplan said, and CDC experts were forced to offer their best guidance without waiting for years of testing and research.

In that situation, after more has been learned, invariably someone will come along and say ``you were stupid and you were wrong,'' Koplan said. ``I think we have done more good than harm.''

The anthrax attacks have also been a sharp learning experience for the Labor Department, according to John L. Henshaw, assistant secretary of labor for occupational safety and health.

``None of us expected bioterrorism in the workplace and we weren't prepared,'' he said.

Postal workers were especially concerned about seemingly conflicting advice given to them after many took antibiotics for 60 days following exposure to anthrax in Washington, New Jersey and elsewhere.

Five people have died of anthrax, including two postal workers, and 13 others have become ill but recovered.

When postal workers finished their first 60 days of antibiotics, CDC experts suggested they could continue on those drugs, add vaccinations, or end treatment. Most opted to avoid the vaccine and District of Columbia health officials discouraged them from taking the shots.

Koplan said he didn't see that as a disagreement, suggesting that after looking at the pros and cons people could simply come to different conclusions.

Asked by workers what he would have done in their case, Koplan said he ``would not hesitate to take the vaccine'' if he had had a significant exposure to the disease.

``But,'' he admitted, ``if I were in your shoes and had taken 60 days of antibiotics and felt lousy from it, my answer might not be the same.''

While that didn't answer all the questions posed by postal workers, it at least eased the concerns of one.

John Ford, a union member who worked at Washington's contaminated Brentwood facility, said he had worried that medical officials were withholding information.

``Now I realize they don't know the answers, period,'' Ford said.

APWU member Jim Burke, who has been active in getting anthrax contamination cleaned up in New Jersey, said this has been the most frustrating five months in his 40 years of work because of the lack of solid knowledge of the disease and how to cope with it.

Koplan did tell the workers that the longer people exposed to anthrax remain symptom-free, the more likely it is that they won't get the disease. But there is no set day the risk is ended, he said.

``With each passing day away from exposure ... you can feel more comfortable that the risk is diminished,'' Koplan said.

The risk varies from person to person depending on how much of the disease they were exposed to.

Experts had originally thought it takes thousands of spores to cause the inhaled form of the disease, Koplan said. Now they know that may be wrong, but still don't know how much exposure is needed for the disease to develop.

The next problem is making sure contaminated buildings are safe and Koplan said officials do not know what a safe level of exposure may be.

There are parts of the country where anthrax occurs naturally and people don't get sick, he noted.

So when it comes to a building in which anthrax spores have been released, ``how clean is clean enough?'' he was asked.

``We'll come up with an answer,'' he said, ``but it'll be a relative one.''