Stichting Fly Aware advises to take the following preventive measures when you take a flight:
Before the flight: get sufficient sleep
During the flight, your body will be facing a lowered air pressure (and consequently less oxygen), possibly a different food pattern than what you're used to, sitting in the same posture for a prolonged period of time, multiple chemical exposures, time differences etc. One of the effects of these stresses is an increased production of free radicals, which can cause a.o. cell damage and accelerated aging. A well rested body can deal with these extra stresses much better, it will improve your resistance against germs and assist the body to get rid of any free radicals and toxic substances it may have collected.
Extra Vitamin C
Before the flight, take extra vitamin C. It is a strong antioxidant that will give the immune systeem a firm boost to eliminate free radicals. During and after the flight, it is also a good idea to take extra vitamin C. Some specialists even recommend to take as much as 1000 mg every hour during the flight. Such extreme doses might cause an upset stomach and diarrhoea, but a slight overdose will not do you any harm. Any unused vitamin C will simply be dumped into urine.
Take your mask with you
Make sure to carry a suitable face mask in your hand luggage. A good quality (activated carbon) mask is produced by Cambridge Masks in the UK. In case of a fume event, these masks will filter out most of the pollution (>99%) in the cabin air. Your health, and those of your travel companions, are certainly worth the investment!
When to wear?
During the whole flight, keep your mask within reach, e.g. in the seat pocket in front of you. Keep in mind that, should a Fume Event occur, you might not get the chance to take your mask out of your luggage in the overhead bins.
Wear your mask at least during engine start, until after Take Off and climb, and also during descent, approach and landing. These are the periods where chances of air contamination by the engines ar highest, as large changes in power settings occur.
On certain destinations, the cabin crew will spray insecticides in the cabin before arrival. This is a legal requirement in several countries, and it is often claimed to be harmless. Don't be fooled there, this is highly toxic material! Wear your mask during these periods, it offers effective protection against inhaling these chemicals.
Furthermore it's obviously wise to wear your mask whenever you observe any "strange" odours ("dirty socks", "wet dog"). You can order your Cambridge "Sky Mask" via this link.
Turn the "blower" off
Once on board, in certain aircraft types you will find a so-called "overhead blower", which can provide you with "fresh" air directly on your face. It is however advisable to turn this off, or at least not to aim it towards your face. Although he air that comes out of it has been filtered (as this is recirculated air that has passed through a so-called "HEPA-filter), and bacteria and viruses are supposed to have been removed, any oil fumes and other chemicals in the air are not filtered out. Via these blowers they will land straight into your face, so you'd better avoid this if possible...