A DOT-approved case is a case that the Department of Transportation approved to transport a lighter safely in your carry-on. Each case must only include one lighter; you can put up to one lighter in each case.
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For the same reason that items with lithium batteries of a certain strength aren't allowed in check-in. They are fire hazards and fire-risk items are much less of a hazard if accessible during flight. Imagine a lighter accidentally igniting in a suitcase in a hold. Not good.
Torch lighters (a.k.a. Butane lighters, Plasma lighters, blue flame lighters, jet flame lighters) are not allowed in the cabin or in checked baggage by both the Hazardous Materials Regulations and TSA security rules.
A prohibition on torch-style lighters, which have hotter flames, will continue. Lighters have been barred from checked bags for decades because of concerns that the lighters might start fires in cargo holds.
TSA is very straightforward that you can bring cigarettes through airport security as a carry-on/personal item and that you can also place them in your checked baggage.
Checked Bags: YesWhile cigar cutters are generally permitted, we recommend that you pack them in your checked baggage. TSA officers have the discretion to prohibit any item through the screening checkpoint if they believe it poses a security threat.
Disposable and Zippo lighters without fuel are allowed in checked bags. Lighters with fuel are prohibited in checked bags, unless they adhere to the Department of Transportation exemption, which allows up to two fueled lighters if properly enclosed in a DOT approved case.
Firearms, ammunition, and fireworks are prohibited, as are all knives and safety razors (including pocket knives and Swiss Army knives). Straight razors and replacement blades for straight razors are also not allowed. Most tools also cannot be packed in carry-on luggage, as they have the potential to cause harm.
Solid food items (not liquids or gels) can be transported in either your carry-on or checked baggage. TSA officers may instruct travelers to separate items from carry-on bags such as foods, powders, and any materials that can clutter bags and obstruct clear images on the X-ray machine.
When a carry-on bag is checked at the gate or at planeside, all spare lithium batteries and power banks must be removed from the bag and kept with the passenger in the aircraft cabin. The battery terminals must be protected from short circuit.