Trees help us breathe — and not just by producing oxygen. Amazon rain forest About ...

... half of all oxygen in the air comes from phytoplankton, but trees are a major source, too. Still, their relevance for humans' oxygen intake is a bit hazy. Various sources suggest a mature, leafy tree produces enough oxygen for two to 10 people per year, but others have countered with significantly lower estimates.

Yet even without the oxygen, trees clearly offer plenty of other benefits, from food, medicine and raw materials to shade, windbreaks and flood control. And, as MNN's Matt Hickman reported in 2016, city trees are "one of the most cost-effective methods of curbing urban air pollution levels and combating the urban heat island effect." That's a big deal, since more than 3 million people die worldwide each year from illnesses linked to air pollution. In the U.S. alone, pollution removal by urban trees is estimated to save 850 lives per year and $6.8 billion in total health care costs.

There's also another notable way trees can indirectly save lives by breathing. They take in carbon dioxide, a natural part of the atmosphere that's now at dangerously high levels due to the burning of fossil fuels. Excess CO2 drives life-threatening climate change by trapping heat on Earth, but trees — especially old-growth forests — provide a valuable check on our CO2 emissions.