When it comes to health, which is more important, nature or nurture? You may well think your genes are a more important predictor of health and ill health. Not so fast. In fact, it transpires that our everyday environment outweighs our genetics, big time, when it comes to measuring our risk of disease. The genome is out—welcome the exposure.

"The exposure represents everything a person is exposed to in the environment, that's not in the genes," says Stephen Rappaport, environmental health scientist at the University of California, Berkeley. That includes stress, diet, lifestyle choices, recreational and medicinal drug use and infections, to name a few. "The big difference is that the exposure changes throughout life as our bodies, diets and lifestyles change," he says.

While our understanding of the human genome has been growing at an exponential rate over the last decade, it is not as helpful as we hoped in predicting diseases. "Genes only contribute 10 percent to the overall disease burden," says Rappaport.

"Knowing genetic risk factors can prove absolutely futile," says Jeremy Nicholson at Imperial College London. He points to work by Nina Paynter at the Brigham and Women's Hospital in Boston, who investigated the effect of 101 genetic markers implicated in heart disease. After following over 19 000 women for 12 years, she found these markers were not able to predict anything about the incidence of heart disease in this group.

On the other hand, the impact of environmental influences is still largely a mystery. "There's an imbalance between our ability to investigate the genome and the environment," says Chris Wild, director of the International Agency for Research on Cancer, who came up with the idea of the exposure.

In reality, most diseases are probably caused by a combination of the two, which is where the exposure comes in. "The idea is to have a comprehensive analysis of a person's full exposure history," says Wild. He hopes a better understanding of exposures will shed a brighter light on disease risk factors.

There are likely to be critical periods of exposure in development. For example, the time from birth to 3 years of age is thought to be particularly important. "We know that this is the time when brain connections are made, and that if you are obese by this age, you'll have problems as an adult," says Nicholson.