What is the healthiest, safest, or most natural tanning method?

how do tanning beds, spray-ons, etc compare? what exactly do they do and are any markedly less-bad for you than others?

how do they compare to the natural tanning process that happens under the sun?

NOTE: i am not a proponent of any of these (so i don’t need answers about how bad intentional/artificial tanning is in general), and am only very curious based on learning that many people i know are using bronzing lotions—and wonder how many of the bizarre tidbits i hear are true or myths created by the industry.