Does the US have the world's cleanest air? No, it's getting dirtier.
This isn't the first time that Trump has falsely touted America's air as number one — he tweeted a similar statement in October 2018, along with a map from a World Health Organization report. The statement was not true then, and it definitely isn't now.
In terms of PM 2.5 pollution — a measure of ultra-fine particulate matter in the air — the country with the world's cleanest air is New Zealand, while the US ranks seventh on that list. Meanwhile, the cleanest cities in the world (in terms of particulate concentrations) are in Sweden.
Air-pollution expert Gabriele Pfister, deputy director at the National Center for Atmospheric Research (NCAR) Atmospheric Chemistry Observations and Modeling Lab, told Business Insider that a single country could never have the "cleanest" air on Earth, because we all live under the same sky.