Exposure to an increased level of air pollutants²（空气污染物） , especially nitrogen dioxide（二氧化氮） , has been associated with lower likelihoods（可能性） of successful pregnancy³ among women undergoing in vitro fertilization（体外受精） , according to a team of fertility researchers. The team examined the outcomes of the first pregnancy attempt of 7,403 women undergoing IVF at Penn State Milton S. Hershey Medical Center, Hershey, Pa.; Shady Grove⁴ Fertility, Rockville, Md.; and Columbia University College of Physicians and Surgeons, New York, N.Y. They conducted their observations over a seven-year period from 2000 to 2007.

"Numerous studies have consistently shown a relationship between air pollution and human health, ranging from mortality, cardiovascular disease（心血管疾病） and other chronic⁵ conditions," said Duanping Liao, Ph.D., professor of epidemiology（流行病学，传染病学） and vice⁶ chair department of public health sciences, Penn State College of Medicine. "In the process of searching for the mechanisms⁷ responsible for the above associations, we, and others, have reported significant links between air pollution and inflammation and increased blood clotting⁸（血液凝结） . These intermediate factors are also associated with reproductive health."

The IVF population was chosen, as it is a well-controlled and timed process to investigate the association of air pollution and human reproductive effects.

Burning of fossil fuels and diesel⁹ engine（柴油机） combustion¹⁰ produces nitrogen dioxide and fine particles. Researchers looked at those pollutants along with ozone¹¹, the gas involved in smog formation. While the effects of declining air quality on IVF success are variable and pollutant¹-dependent, elevated exposures to nitrogen dioxide and fine particles were consistently associated with lower success rates of pregnancy.

Researchers looked at the effects of pollution particles both individually -- single pollutant model, and with other particles -- multi-pollutant model. For the single pollutant model, exposure to ozone appeared to have a positive association with a successful birth if the exposure was before the embryo¹²（胚胎，胚芽） culture or embryo transfer. Researchers theorize（推理） that higher ozone levels indicate lower nitrogen dioxide levels, which would show better pregnancy outcomes. In addition, for the multi-pollutant model, the "positive" effects of ozone were diminished（减少，削弱） with the addition of nitrogen dioxide. In contrast, after adjusting for ozone, higher nitrogen dioxide exposures consistently associated with the lower success rate, regardless of which indictor was used -- positive pregnancy test, clinically confirmed intrauterine（子宫内的） pregnancy or live birth.

These findings may be useful in studying the adverse¹³ effects of air pollution on human reproduction in general.

"Since IVF is a well controlled and highly timed process, we have a much better handle on the assessment¹⁴ of the time of exposures to elevated（提高） air pollutants in relationship to fertilization, pregnancy, and delivery," Liao said. "Therefore, the IVF population coupled with detailed¹⁵ assessment of air pollution exposures may provide us an ideal situation to investigate the potential health effects of air quality on human reproduction."

Air pollutant concentration data for the study period came from the U.S. Environmental Protection Agency. The researchers calculated daily pollution concentrations for each patient during the entire in vitro cycle and pregnancy. They calculated varying periods of air quality exposure for average daily concentrations at the patient's home during four stages of IVF and at the IVF clinic during fertilization and embryo transfer.