Wetsuits help to preserve body heat by trapping a layer of water against the skin; this water is consequently warmed by body heat and acts as an insulator. Water conducts heat away from the body approximately 25 times more efficiently than air, so an unprotected individual can succumb to hypothermia even in warm water on a warm day. Wetsuits are made out of closed-cell, foam neoprene, a synthetic rubber that contains small bubbles of nitrogen gas when made for use as wetsuit material. Nitrogen gas has very low thermal conductivity, so it does not allow heat from the body (or the water trapped between the body and the wetsuit) to be lost to the water outside of the wetsuit. Some wetsuits of late have even begun to incorporate thermal fabric and titaniun fibers to add warmth characteristics, while keeping the thickness of the suit to a minimum. A wetsuit must have a snug fit to work efficiently; too loose a fit will allow water to escape from between the suit and the body, taking the body's heat with it. Flexible seals at the suit cuffs aid in the water retention. Neoprene is very buoyant, helping swimmers to better stay afloat, and for this reason divers need to calculate extra weight values based on the thickness of their suit to achieve neutral buoyancy underwater. The suit loses buoyancy and thermal protection as the bubbles in the neoprene are compressed at depth.