As long as scientists have been making babies in petri dishes, they have tried to do in a lab what a woman's body does on its own: keep fragile embryos safe, warm and protected from toxins.

Last week, the Duke University Medical Center opened a $6 million in vitro fertilization lab devised to the smallest detail to rival Mother Nature's elegant design. Entrances to the lab and procedure rooms -- even the electrical outlets -- are sealed to block airborne pathogens. The paint on the walls was chosen for its minimal output of noxious fumes. Special countertops were used.

"There's an element of paranoia to this," said Dr. David Walmer, chief of Duke's division of reproductive endocrinology and fertility. "But we all know there are chemicals in the environment that are harmful [to embryos]. The best thing you can do is actually seal the lab and make sure nothing is leaking in."

Of the more than 400 in vitro labs in the United States, only about a dozen go to such lengths to maintain air quality.

Duke hopes a lab free of airborne toxins, which are known to slow the growth of embryos or even kill them before they are put back in the mother's womb, will increase its success at helping infertile couples conceive. Some medical centers, including the Cleveland Clinic in Ohio, have raised pregnancy rates significantly after switching to sealed in vitro labs.

Higher pregnancy rates mean more couples conceive on the first try, reducing the need for repeated sessions. That's important because the technique typically costs $8,000 to $10,000 a pop -- and most people pay out-of-pocket because health insurance often doesn't cover it. About 32 percent of women younger than 35 became pregnant after receiving in vitro at Duke in 2003, according to the Centers for Disease Control and Prevention.

The link between clean air and successful in vitro isn't new.

Jacob Mayer, director of the Virginia lab that in 1981 produced the United States' first "test tube" baby, published groundbreaking research on the subject in 1999. His work showed that embryos in incubators with air purifiers more readily attached to the mother's womb -- resulting in a pregnancy -- than embryos cultivated in incubators without purifiers.

"In hindsight, it makes sense," said Mayer, director of the in vitro lab at Jones Institute for Reproductive Medicine, in Norfolk. "In some ways our early attempts, while state-of-the-art for the time, were crude."

Like nature intended

When an egg is fertilized in a woman's body, natural defenses keep the embryo safe. Mom maintains the proper temperature and acidity, and if she comes in contact with paint fumes or air pollution, her liver and lungs filter them out.

Embryos that start in a petri dish don't have those natural protections. But efforts to keep out chemical contaminants have evolved. Fertility centers now routinely use air purifiers and filters to scrub the environment in and around incubators. And many labs try to minimize airborne contaminants when they make improvements.

UNC Hospitals in Chapel Hill, for example, incorporated some of the same features in Duke's new lab when it built the in vitro lab in its N.C. Women's Hospital, which opened in 2001. Lab director Stan Beyler made sure he got solid ceilings and chose special flooring and paint. Hugh Hensleigh, director of the in vitro lab at the N.C. Center for Reproductive Medicine in Cary, sought nontoxic paint when the center remodeled last year.

But even the most meticulously controlled lab can't stop the hospital department down the hall from sprucing up its area, kicking up dust and chemicals. It can't keep fumes from wafting in when cleaning crews break out the ammonia.

"Back in our old lab, if someone decided to paint their hallway, or if they were spraying for bugs, we were sort of at their mercy," said Doug Raburn, director of Duke's in vitro lab.

Duke worked with a consultant that specializes in building sealed in vitro labs, Alpha Environmental of New Jersey, to design its 12,000-square-foot fertility center near the Streets at Southpoint mall in Durham.

The new lab and procedure rooms, where eggs are extracted from ovaries and embryos are put back into wombs, are designed to bring clean air in and keep polluted air out. Purified air is pumped into the lab to create "positive pressure" -- a technique often used in operating rooms and other sterile areas, causing clean air to push out contaminants that try to seep in.

To discourage other leaks, walls and ceilings are solid, not built from porous materials often used in offices. Electrical outlets and ducts are sealed. Countertops and cabinets are fashioned from steel rather than Formica or particle board, which vent chemicals such as formaldehyde. Walls are painted with water-based paint, tested to ensure low output of fumes.

Many, though not all, of those features are in most in vitro labs, said Beyler, the UNC lab director. He helped write the national standards in vitro labs must meet to win accreditation.

"Some of it may be a little bit of overkill," Beyler said of the Duke lab. "They're publicizing every little detail, and that sounds impressive. And it is, frankly. It's always good to err on the side of caution."