Mona Lisa's smile remains one of art's great mysteries, and many credit it with the portrait's enduring appeal.

But her expression can't explain everything, said Austrian neurologists who used computer-altered images to simulate the "Mona Lisa condition" in a new study.

Job-related eye strain and injuries cost the American economy billions each year in lost productivity, but 90 percent of those problems could be prevented with simple measures, such as having workers wear properly designed and fitted protective eyewear, experts say.

"Healthy vision is critical to successfully completing job-related tasks," James Sheedy, director of the Vision Ergonomics Laboratory at the College of Optometry at Pacific University and the American Optometric Association's (AOA) occupational vision specialist, said in a news release.

Running a marathon is challenging enough, but now new research shows that the performance of female marathoners can be hindered by a certain type of air pollution.

Researchers analyzed marathon race results, weather data and air pollution concentrations in seven major U.S. marathons over a period of eight to 28 years. They found that higher levels of air pollution particles were associated with slower finish times for women. Air pollution levels didn't appear to have a significant impact on men, the study authors noted.

When the stock market declines, do heart attacks go up? That's what Duke University Medical Center researchers are wondering based on an analysis of data collected during the current U.S. economic crisis.

"During the period that the NASDAQ was declining, the MI (myocardial infarction, or heart attack) rates were increasing," Mona Fiuzat, the study's lead investigator, said in a university news release.

MRI scans are more likely to turn up undiagnosed tumors in the breasts of postmenopausal women who already had cancer in their other breast, researchers report.

However, the scans are less likely to detect tumors in premenopausal women, they added.