About one couple in 20 that depends on condoms for birth control or disease prevention is finding that the condoms break at least 20 percent of the time, according to studies in the United States and abroad.
The studies, examining the role of human behavior in condom failures, found that condoms break most frequently when couples use oil-based lubricants, attempt their own "quality testing," or engage in prolonged sex, said Carol Joanis, of Family Health International (FHI), a non-profit family planning research agency in North Carolina.
Some breakage also is linked to the condoms' age, or exposure to heat, humidity, light and air pollution, which weaken the latex from which most condoms are made.
The high rates of condom failure -- as high as 60 percent for some individuals -- were found among 4 to 6 percent of all condom users in the studies, regardless of age, culture or type of condom.
Failure rates are a worry because, next to abstinence, condoms are regarded as the first line of defense against sexually transmitted diseases, including AIDS. Condom sales rose 60 percent during the late 1980s as concern about AIDS grew.
On the whole, Ms. Joanis said, "condoms are a very reliable product."
The studies completed last December found failure rates ranging from zero to 13 percent. Health officials stress that sexually active people are always far safer with them than without.
"What we have learned. . . is that condom breakage, by and large, occurs with a fairly select group of people," Ms. Joanis said.
Dr. Ronald H. Gray, of the Johns Hopkins School of Hygiene and Public Health, said some people do become pregnant and sick because of condom breakage. But the failure to use condoms consistently, and condoms that fall off during intercourse are probably bigger risk factors than breakage. "These things are really very difficult to study," he said.
In a set of studies conducted over two years, FHI examined sexual practices and condom performance among 1,700 people -- 300 in Raleigh-Durham, N.C., and 1,400 in eight foreign countries.
They found most couples aren't very tough on condoms, Ms. Joanis said. Married or cohabiting couples in the study broke 1 to 2 percent.
"When you get to casual sex, extramarital affairs or lovers, you get a much more vigorous sex act and a higher breakage rate -- 3 to 5 percent," Ms. Joanis said. That's unfortunate, because "these are the people that need the highest protection."
Overall, she said, "the breakage rates range from 0 to 13 percent [during vaginal intercourse] . . . across every culture, country and type of condom."
Other studies have found that anal sex produces higher condom failure rates.
But FHI found that 4 to 6 percent of people in the studies reported they broke more than 20 percent of the condoms they used. Interviewers found several important patterns among these so-called "breakers."
One of the most common mistakes people make, despite package warnings, is using hand lotion, baby oil, Vaseline or some other oil-based product as a lubricant. Laboratory tests have found that oil-based products can reduce a latex condom's strength by 90 percent in as little as 60 seconds.
Eli Carter, senior project manager at FHI, said oil breaks down the cross-linking of molecules that gives latex its strength, "making the latex very weak."
If couples need lubricants, Ms. Joanis said, they should buy water- or silicone-based products designed for such use.
Interviews with people reporting the highest breakage rates have found that many of them unroll, stretch and blow up their condoms to "test" for leaks before using them. Some also pull them on like socks.
"That just weakens them," Ms. Joanis said. Condoms should be unrolled directly and completely onto the penis. Never reuse them.
More effective testing -- water leakage and tensile strength tests -- is done by manufacturers at the factory, Mr. Carter said, following standards set by the U.S. Food and Drug Administration. The FDA also periodically tests condoms made in this country and abroad.
Current FDA regulations allow no more than four weak or leaky condoms in 1,000. If more fail, the entire lot must be discarded.
"All condoms coming into this country must meet these standards also," Mr. Carter said.
Most U.S. manufacturers also add airburst tests, measuring the air that can be blown into a condom before it breaks. Airburst tests aren't required by the FDA, but they are by many countries where U.S. manufacturers sell condoms.
In the first year after the FDA rules took effect in 1987, FDA tests on 115,000 condoms led to the recall of 3 million domestic condoms and rejection of 30 million imported condoms. Five foreign manufacturers were barred from further shipments to the U.S.
A federally funded study at UCLA in 1989 found that 6.6 condoms in 1,000 failed water, tensile or airburst tests. That study, and another by Consumer Reports magazine in March 1989, found sharp differences among brands.