An international team of researchers led by MIT has explained how contaminated batches of the blood-thinner heparin were able to slip past traditional safety screens and kill dozens of patients recently in the United States and Germany.

The team, led by Professor Ram Sasisekharan of MIT, identified the chemical structure of the contaminant, known as oversulfated chondroitin sulfate (OSCS). The researchers present their findings and offer new approaches to detecting the contaminant in a report appearing today in the online edition of Nature Biotechnology.

Another team led by Sasisekharan has shown exactly how OSCS can kill--specifically by setting off an allergy-like reaction. The biological effects of the contaminant are outlined in a report also being published online today in the New England Journal of Medicine.

In fury and despair, people harmed by Lasik eye surgery told of severe eye pain, blurred vision and even a son's suicide, as the government began considering if the public needs more warning about the risks of the hugely popular operations.

About 700,000 Americans a year undergo the elective laser surgery. Like golf star and famed Lasik recipient Tiger Woods, they're hoping to throw away their glasses, just as the ads say.

And while the vast majority benefit - most see 20-20 or even better - about one in four people who seeks Lasik is not a good candidate, and a small fraction suffer serious, life-changing side effects: worse vision, severe dry eye, glare, inability to drive at night.

"Too many Americans have been harmed by this procedure and it's about time this message was heard," David Shell of Washington told the Food and Drug Administration's scientific advisers Friday.

Shell held up large photographs that he said depict his blurred world, showing halos around objects and double vision, since his 1998 Lasik.