Junk food, lack of exercise and now even the air you breathe could contribute to you gaining weight. Danish researchers have proposed a theory that steadily rising levels of carbon dioxide in the air maybe increasing appetite and could explain the rising obesity rate.

The theory goes that breathing in extra carbon dioxide makes the blood more acidic which affects the hormones in the brain that influence energy expenditure and food intake. As a result people are eating more and gaining more weight because of the air they breathe.

The theory is being explored by Lars-Georg Hersoug, a scientist from the Research Centre for Prevention and Health at Glostrup University Hospital in Denmark. According to Hersoug the rise in obesity in the U.S. was the fastest in the period from 1968 to 2010 on the East Coast where CO2 concentrations were the highest. Animal tests also proved that environmental factors also contributed to weight gain that inhaling carbon dioxide makes the blood more acidic.

"The normal theory is that fat people get fatter because they don't move as much as they should," Hersoug told Science Nordic. "But the study showed that thin people also get fatter, and this happened over the whole of the 22-year period of the study."

In 2011, Hersoug began testing his hypothesis with researchers Anders Mikael Sjödin and Arne Astrup from the University of Copenhagen. They tested six young men to see how CO2 would affect their appetites. They place the men in different special climate rooms where some of them were exposed to increased amounts of carbon dioxide. After seven hours the men were allowed to eat as much as they liked.

The researchers found that the men who stayed in the room with greater amounts of CO2 ate six percent more food than the men who stayed in the room with a normal amount of CO2.

"We could also see that the extra amount of CO2 caused the men's heartbeat to rise, and this gives us an indication that CO2 affects the brain's nerve cells - orexins in the hypothalamus - which among other functions control our appetite and the composition of our nutrient intake," said Hersoug. "A very small change in the activity of these nerve cells will presumably have great importance for our development of obesity or for maintaining our weight."