What are the things that are banned by airlines on planes?

Is it true airlines use recycled air?

• I have been told, airlines stooped fully air conditioning planes, after smoking was banned. hence have become a breeding ground for germs.

• Answer:

  You have been told incorrectly. Most modern airliners use a mix of outside air and recirculated air for air conditioning. This has been true since long before smoking was banned. About half the air pumped into the cabin comes directly from the outside; the other half is taken from the cabin, pumped through HEPA filters, and sent back into the cabin. The HEPA filters are very efficient and remove germs from the air that is recirculated. There are two reasons for recirculated part of the air: (1) the air from the outside is pumped in using engine power to compress and heat it, and this reduces the amount of engine power available for flying the airplane, so recirculating part of the air reduces the load on the engines; and (2) air from the outside is extremely dry, whereas recirculated air is more humid, and the extra humidity from the recirculated air makes the cabin atmosphere more comfortable for people. The air conditioning system is thus not a "breeding ground for germs" by any stretch of the imagination. The air in the cabin is considerably cleaner than air on the ground, since it doesn't contain pollution, pollen, or germs. While it is true that airlines are slow to replace the air filters, that doesn't make the air dirtier. A clogged air filter actually traps more dirt, not less, since it's harder for air to pass through it. The only disadvantage to a dirty filter is that it slows the air flow. When people get sick after flying somewhere, it's not because of anything wrong with the air in the airplane, it's just that they are exposed to different germs when they travel to different places. Since infections like the common cold are caused by hundreds of different viruses, a person traveling is more likely to be exposed to a virus that his body hasn't seen before, and thus he's more likely to come down with something. But airplanes have nothing to do with that. In fact, air circulation in most airliners is vertical rather than horizontal, so even if someone with tuberculosis is on the plane, the only people at risk are the ones sitting next to him in adjacent seats. And the filters in the air-conditioning system stop tuberculosis germs, too.