Vitamin D is vital to our lives and is the first defense against such ailments as cancer, osteoporosis, diabetes, multiple sclerosis, and other immune system diseases. We need only 15 to 35 minutes a day of outdoor exposure to sunlight three or four times per week to generate an adequate amount of vitamin D, which is estimated to be between 120 and 150 nanomoles per litre of blood. Many of us, however, do not receive adequate exposure to sunlight because of climate, working conditions, health problems, or age. Some of us are bedridden or old and not very mobile, thereby limiting our exposure to sunlight. Because glass filters out about 95% of the UV-B radiation, the length of exposure to sunlight indoors must be nine or ten times longer than the recommended outdoor exposure. Buildings that house the elderly have even higher requirements for sunlight because aged skin diminishes the cutaneous photosynthesis of vitamin D.