Fluorides are compounds that combine the element fluorine with another substance, usually a metal. Examples include sodium fluoride, stannous fluoride, and fluoride monofluorophosphate (MFP fluoride). Some fluorides occur naturally in soil, air, water, or plants. Once inside the body, fluorides are absorbed into the blood through the digestive tract. They travel through blood and tend to collect in areas high in calcium, such as the bones and teeth. The US Environmental Protection Agency (EPA) has set a maximum amount of fluoride allowable in drinking water of 4.0 mg/L. Long-term exposure to levels higher than this can cause a condition called skeletal fluorosis, in which fluoride builds up in the bones. This can eventually result in joint stiffness and pain, and can also lead to weak bones or fractures in older adults.