There is no open flame. They are completely harmless and stay toasty warm for 24 hours..Kudos, I think, to @riverrun for suggesting this brilliant edition to a winter EDC: http://www.hakkin.co.jp/
Sure, it's a small, heated, white gasoline filled metal container...stuffed into your pocket...but it's been in use since 1923 and not too many people have died. I actually really like it!
You can even pick some up - vintage Jon-e or modern asian - for less than $5
There's a slew of them out there!