The next time you read Outside in the shade of a leafy tree, recognize that you're in the presence of greatness. The USDA Forest Service recently discovered that trees are responsible for saving more than 850 human lives and staving off 670,000 incidents of acute respiratory systems annually.

Trees are valuable not only for what they give us—wood, oxygen, fruit—but also for what they take away. The air is full of major pollutants, such as nitrogen dioxide, ozone, and sulfur dioxide, but through a process called deposition, pollutants that come in contact with tree leaves get "deposited" onto them or absorbed into the stomata. The more particulates that end up on or in leaves, the less concentrated they are in the air, and fewer end up in our lungs. Even with trees, about 130,000 Americans die in part because of air pollution every year.

Forest Service researchers set up computer simulations with county-level environmental data from 2010 to understand just how effective trees are at hoovering particulates across the United States. Researchers looked at the amount of tree cover in an area, plus the hourly exchange of pollutants between leaves and air and its effects on pollution removal on the atmosphere, and compared that data to the health impacts and monetary value gained from the decrease in ozone, nitrogen dioxide, and other pollutants.

The researchers concluded that the lives and lungs saved by tree-reduced air pollution—which, at 17.4 million tons, accounts for less than 1 percent of total annual air-quality improvement—equates to nearly $7 billion each year, particularly in urban areas. While deposition is greater in lusher rural areas, trees save more lives through this process in urban areas, which the researchers say underscores the importance of urban forests.

Curious about just how many times trees kept you from missing a day of work or dying? Check out the tables in the report.