According to the EPA ozone research, breathing in ozone can cause vascular system inflammation, a change in heart rate variability, and a reduction in the capacity of blood clots to dissolve, all of which are risk factors for heart disease. However not all studies agree that these effects exist. Some studies have found no evidence of this relationship between ozone and heart disease. The science is still out on how much ozone people breathe in over time and what health effects it may have. But based on what we know now about its danger to humans, it's reasonable to assume that breathing in more than 0.075 parts per million of ozone for several hours each day could be harmful. We know that air pollution is responsible for more than 7 million deaths a year. That's almost as many deaths as smoking. Ozone is one of those pollutants that we can actually see in the air: It looks like white clouds when it floats above trees or buildings. Because of this, everyone is exposed to ozone, but especially those who live in cities where traffic fumes, power plants, and other sources of pollution are close by. We also know that smokers have more likely to suffer heart attacks and strokes, and researchers think that this might be because they absorb more of the toxic substances in cigarette smoke than do non-smokers. This extra exposure could also increase their risk of developing lung cancer.