Care should be taken when winter swimming in swimming pools and seas near the polar regions. The chlorine added to water in swimming pools and the salt in seawater allow the water to remain liquid at sub-zero temperatures. Swimming in such water is significantly more challenging and dangerous. The experienced winter swimmer Lewis Gordon Pugh swam near the North Pole in water and suffered a frostbite injury in his fingers. It took him four months to regain sensation in his hands.