While simply stepping outside in cold weather without a jacket doesn't cause a cold, hypothermia (the lowering of the body's core temperature) suppresses immunity, which can lead to colds. "Most cold symptoms are produced by the body's immune system physically responding to the rhinovirus," says Belilovsky. "So, someone with a stronger immune system in the cold will produce more [mucus], while the one with the weaker immune system will sniffle longer, but less dramatically." The person with the weaker immune system probably will have more complications, such as sinusitis or ear infections, Belilovsky adds.
Cold weather may be indirectly responsible for colds, however. Vasoconstriction, when blood vessels close to the outside of the body narrow, such as those found in the nose, leads to dryness. "This dryness compromises the nose's ability to filter infections," Belilovsky explains. "On returning to warm air, rebound vasodilation occurs, where your hands get pink and your nose starts running as blood returns to it." The cycle continues if the runny nose is severe enough to cause mouth breathing. Bypassing the nose's ability to filter inhaled air, combined with dry indoor air, allows the inhalation of virus-bearing mucus, which may trigger colds and lower respiratory infections.