Butane which is used in disposable bic lighters is very safe from what I understand. The byproducts of combustion is not much more that simple water vapor and CO2. Naphtha on the other hand, which is used in wick type lighters like Zippo, I think is less benign. It doesn't seem to burn as cleanly, which means you get more unburnt hydrocarbons in the fumes. You can tell the difference with your nose when lighting the two types. Zippos have much more smell, while bics have none.