Causes of headaches: Car Fumes
Exactly the same principles apply to car exhaust fumes as apply to gas. Incompletely burning petrol gives off carbon monoxide as a waste product, and even the best maintained car engines emit a certain amount into the atmosphere. If operated in an enclosed space (such as a garage with the doors shut) there is a quick build-up of carbon monoxide in the air. This is the reason why you should never start a car engine without first opening the main garage door. Any leakage of exhaust gas into the passenger compartment can create headaches through carbon monoxide poisoning. The level of exhaust gases in the passenger compartment can often be relatively high and if you're aware that your car always smells of exhaust fumes after a journey, then you are probably exposing yourself to progressive amounts of carbon monoxide. If you end up with a headache and nausea alter every car journey this is additional evidence that you've got exhaust-leakage problems.
As well as carbon monoxide poisoning, you can also become sensitive to other chemicals liberated in the exhaust.
Self-help
Be aware of the potential problem. Check your exhaust pipe regularly and make sure that any holes or leaks are mended as soon as they're found. In addition, make sure that the engine is properly tuned. This reduces carbon monoxide emission to the lowest possible levels.
One particularly awkward situation concerns people who live or work in areas with a high degree of traffic pollution - those who work in underground car parks, near big roundabouts where the vehicles are likely to be stuck in traffic jams for long periods of time, or in narrow and enclosed streets that are poorly ventilated by the wind. Again the cure is quite simple - identify the cause of the problem, and avoid exposure to the offending substance. On the other hand, it is unusual to have problems with car fumes unless you are working in an enclosed space. Making sure there is good ventilation and, if necessary, installing decent extractor fans, can make a lot of difference.
Don't forget that you can become sensitive to petrol itself (see solvents, below). If you're getting headaches and there's no obvious leakage of exhaust gases into the car, but there is a smell of petrol, it may be that you are sensitive to the petrol rather than being poisoned by carbon monoxide. If your petrol tank is slightly leaky, petrol fumes may well drift into the passenger compartment - especially in hatchbacks, and cars with back seats that let down to allow a continuous space from the passenger compartment into the boot. Because the petrol tank is often situated underneath the boot, any leakage of petrol vapour into the boot in cars like these can quickly circulate into the passenger compartment. If you frequently smell petrol, get the engine and petrol tank checked out.
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Causes of headaches: Food, Chemicals, Drugs, Additives and Alcohol

Causes of headaches: Gas