Causes of headaches: Gas
In the old days, town gas used to contain high quantities of carbon monoxide. These days, in Britain, we use North Sea (natural) gas almost exclusively. This type of gas contains methane, but no carbon monoxide. Methane is not a direct poison, though in high enough concentrations it can be dangerous simply because it can displace the oxygen from the air - during a large gas leak, for example. Inhaling pure natural gas would be fatal simply because it doesn't contain any oxygen.
In order for gas to burn properly, a good supply of oxygen is needed. If there's not enough oxygen the gas burns incompletely. When methane burns in a good supply of air it forms waste products of water (as steam) and carbon dioxide. However, if burning takes place in a poor air supply carbon monoxide will be formed.
Carbon monoxide is very poisonous; it poisons the system by interfering with the way oxygen is transported from the lungs to the tissues. Therefore you must ensure that your gas fire has an adequate supply of air, and a good, clean vent. The importance of not blocking up the ventilation bricks in a room with a gas fire is obvious. In small quantities it merely reduces the oxygen-carrying capacity of the blood, but in larger amounts it can kill. Continual exposure even to low levels of carbon monoxide can quickly poison a very large percentage of the oxygen-carrying capacity of the blood.
Carbon monoxide poisoning is not a permanent thing, and in small quantities it debilitates rather than kills. However, it is a potentially vicious killer, and the more carbon monoxide to which you are exposed at any one time, the greater your likelihood of dying. The symptoms of carbon monoxide poisoning are principally headache, drowsiness and befuddlement.
Carbon monoxide is also present in cigarette smoke. The amounts are too small to kill, but it definitely impairs the ability of the body to supply the tissues with oxygen.

Orthodox treatment
Carbon monoxide poisoning is potentially fatal. If there is any question about the health of the patient - and especially where a young child or an older person is involved - you should call a doctor immediately. The patient may need to go to the hospital. In hospital, treatment is simple - giving high concentrations of oxygen increases the amount of oxygen carried by the blood (although the red blood cells which normally carry oxygen have teen poisoned by the carbon monoxide, some oxygen goes along dissolved in the blood serum) and after a while the carbon monoxide dissociates from the red blood cells, which can then go about their oxygen-carrying work normally again. After a minor degree of exposure, the patient is usually back to full health in a couple of days. However, lack of oxygen quickly and often permanently damages the brain, and death or severe brain damage can follow in severe cases of carbon monoxide poisoning, as well as damage to other tissues due to lack of oxygen.
Self help
Don't close off ventilation holes in rooms with gas fires. Do get your gas fires serviced regularly. Do check the chimney flue for birds' nests and anything else that can cause pressure, which would force waste gases back into the room.
Don t use old-style open-Hue water heaters (the type that get their supply of air from within the room) in bathrooms and other enclosed spaces. A sudden, massive build-up of carbon monoxide like this can kill the occupant in the bath. Closed-system burners are much safer. These are the so-called 'balanced flue' systems, where incoming air is drawn exclusively from the outside, and the exhaust gases are vented exclusively to the outside.
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Causes of headaches: Food, Chemicals, Drugs, Additives and Alcohol