Some are obvious, but some are surprises. During flight, changes in temperature and pressure may cause items to leak, generate toxic fumes or ignite. Many common household items can become hazardous when exposed to high altitudes. Contact your airline if you think you’re bringing anything questionable.. Under federal law, you must declare with the carrier, any possibly hazardous items that you are carrying or shipping. Violations may carry large fines and/or  prison time. No list is all-inclusive and each airline is allowed to develop its own restrictions. They may be more or less restrictive than the requirements of the F.A. A.

• All flammable liquids : Fuel, paints, paint-thinners/cleaners, lighter fluid, butane fuel, curling iron refills, lighters with flammable liquid reservoirs.

• Any flammable solid such as "easy strike" matches, fireworks, signal flares, sparklers, ammunition, gunpowder or other explosives.

• Bleach, drain cleaners, solvents, corrosives and oxidizers.

• Pressurized containers such as spray cans (hair spray, deodorant or repellents).

• Recreational items such as scuba tanks, propane tanks, CO2 cartridges, self-inflating rafts and camping equipment with fuel.

• Dry ice, gas-powered tools, wet-cell batteries, oxygen tanks, radioactive materials, poisons and infectious substances.

• Any item that might be used as a weapon. This would include firearms, mace, tear gas, pepper spray, knifes, cutting instruments etc.

Yes.  Some personal care, medical necessities, sporting equipment and items to assist physically challenged passengers may be seen as an exception. Contact the airline for its exact policies and any fees that may apply.

• Some unloaded firearms may be allowed in checked baggage, if locked inside secure cases. Contact the airline for its individual guidelines in this regard.  Also check the laws concerning carrying a gun to the airport and to your destination. A certain amount of boxed, small arms ammunition, for personal use may be transported in checked luggage.  Again, check beforehand as amounts will vary depending on the airline.

• Some personal hygiene items, such as perfume containing hazardous chemicals may be allowed on board, but they are limited to no more than a 16 oz. container and a total of no more than 70 ounces overall.

• Dry Ice, 4 lbs. or less, for packing perishables, can be carried in the cabin, provided the package is vented.

• A scuba tank that has been drained to a low amount of psi compression will be allowed by most carriers.

• Electric wheelchairs will be accommodated on board. The battery will need to be disconnected, removed and the terminals insulated to prevent short circuits.

• Most airlines provide supplemental, medical oxygen with proper documentation of the medical need. Advance notice must be made for in-flight use only. Most don't provide it for use on the ground.

• Since many bags are similar in appearance, someone could accidentally pick up your bag in error.

• Often professional thieves work most airports and wait for a turned head to make their move.

• Someone could put something illegal in your bag.

• Worse, someone might place an explosive device in your luggage.

• Security will ask if you packed your own bags or if you left them unattended at anytime. If you did leave them unattended, security will want to take the time to search them thoroughly. 

• In the interest of safety, they could even refuse to allow the bags on the plane.

Yes. Honesty is definitely important and security questions should be answered honestly and completely. Honest answers could save lives ! If you left luggage unattended or even just looked away for a few seconds, someone could have put something dangerous inside a bag. If someone asked you carry something on the plane, it could be dangerous or prohibited. Not just bombs but narcotics or smuggler's contraband could get you in trouble. For everyone's safety, never leave your bags unattended or ever accept anything from strangers. Complete candor is important. If you did happen to leave your baggage alone at any time, admitting that you did will only mean a more thorough security check. Since you'll be on the plane it makes sense that you’ll want to make sure they do !

The questions are meant to locate dangerous items that have been placed with innocent travelers. If you, or someone close to you  packed your bags, and you have always watched and have not accepted any items from strangers, it's much less likely that your bags are carrying dangerous items. At check-in, you will be asked questions such as: Did you pack your own luggage? Did you leave your luggage unattended at anytime ?  Has anyone asked you to carry items on this flight ?  Are you carrying any hazardous materials ? These questions are meant to help keep everyone safe.Of course anyone could just lie and the questions alone won't catch someone who has planned to bring something dangerous on board.