Natural background radiation is high in certain parts of the world, and there is a risk of food contamination from this source. Certain fish can concentrate heavy metals, and will concentrate radioisotopes of these metals in the same way. Some marine animals (for example, molluscs) have dangerously highly levels, concentrated mainly in the shells or bones of the animals but also in their edible portions. For example, radioisotopes have been accumulated by oysters at levels sufficient to present a hazard from their consumption as food. Fish such as salmon, tuna, and anchovy also accumulate sufficient heavy metals for high levels of consumption to result in an increase in the body-burden of radioactivity. Frequent air travellers like airline pilots are exposed to cosmic radiation. Medical exposure through X-rays and barium enemas account for other forms of exposure. Radon is the biggest hazard. It seeps up through the soil from radioactive rocks, especially granite. Because the air pressure in houses is slightly lower than outside, they tend to suck in radon from the ground. Radon is the biggest single cause of lung cancer after smoking.