From the January 6, 2020, business section of the Chronicle.

A story by Mallory Moench in the San Francisco Chronicle lays it out:

Running a lawn mower for an hour generates as much smog-forming pollution as driving a 2017 Toyota Camry from Los Angeles to Las Vegas, according to the California Air Resources Board, which works to keep the air clean. A leaf blower is worse — all the way to Denver. Daily exposure to the fumes also increases cancer risks, a 2018 air board study found.
“The reason that they’re such high polluters, there’s not anything fundamentally different about engines, they’re not fundamentally dirtier, but we haven’t put effort into cleaning them up like cars,” said Dorothy Fibiger, an engineer with the air board’s Monitoring and Laboratory Division. Because some machines such as leaf blowers are handheld, they can’t take on added weight for equipment — like the catalytic converters carried by cars — that reduces emissions, she explained.

The whole story is worth reading. As mentioned in this Atlantic piece from last year, California is the only state that is legally allowed to regulate lawn equipment on air-pollution grounds—rather than on the basis of noise control and other public health reasons. But action against this technologically obsolete, gratuitously polluting equipment is spreading rapidly.