Since we are all exposed to the same atmospheric oxygen and food (which have constant isotopic ratios), the isotopic ratios of drinking water determine the ratios in hair (the ratio in hair is not the...Since we are all exposed to the same atmospheric oxygen and food (which have constant isotopic ratios), the isotopic ratios of drinking water determine the ratios in hair (the ratio in hair is not the same, but is proportional to, the drinking water ratio).