One day after quitting, carbon monoxide levels drop and oxygen levels increase, which reduce the risk of a heart attack
Boffetta P., Hecht S., Gray N., Gupta P., and Straif K., Smokeless Tobacco and Cancer, The Lancet Oncology
As all subjects were scanned at essentially the same age, the rationale is that for a given amount of lifetime smoking (that is, pack-years), if the possible effect of smoking is at least partially reversible, then those who stopped smoking long ago should have a thicker cortex than those who stopped smoking more recently or are currently smoking
New reporting from the journal Addictive Behaviors shows that between 2015 and 2022, the number of Americans smoking blunts either daily or occasionally rose by 24% and 34% respectively