Sunstroke is another name for heatstroke, a medical emergency that occurs when the body overheats to the point that it can no longer regulate its internal temperature. Sun poisoning is a layman's term for the symptoms that can accompany a bad sunburn - nausea, vomiting, fever, chills, headache and, in general, feeling pretty sick.
Some people are overly sensitive to sun exposure - in effect, they have a sun "allergy," usually an itchy rash that develops as a result of an immune system reaction to sunlight. Some of these reactions are inherited and some may be worsened by medications, including certain antibiotics or herbs such as St. John's wort. They also can be due to the interaction of the sun's rays with perfumes, soaps and other substances on your skin. A severe sunburn can be the start of this problem, which can increase over time until people cannot tolerate any sun exposure at all.
I'm sure that everyone knows that repeated, excessive sun exposure can lead to skin cancer, both the more common curable types and the more dangerous, potentially life-threatening melanoma. Sun damage can also cause cataracts to develop in the eyes as well as other types of eye problems including degeneration of the macula, the part of the retina where visual perception is most acute. Always wear sunglasses that block 99-100 percent of UV rays. In addition, overexposure to ultraviolet radiation from the sun can suppress proper functioning of the body's immune system.
The best ways to protect yourself from sun-related problems are to stay out of the sun when it's at a high angle in the sky, to wear hats and protective clothing, and to use sunscreen with an SPF of 30 or higher (apply it at least 15 minutes before going out into the sun and reapply it every two hours). Choose a sunscreen containing Parsol 1789, an ingredient that protects against both ultraviolet A rays that cause sun damage and deep skin wrinkling, as well as the ultraviolet B rays responsible for sunburn and skin cancer.
Having said that, I recommend trying to enjoy some time in the sun every day without sunscreen protection in order to optimize levels of vitamin D, which is made in the body with exposure to sunlight. In the U.S. latitudes south of Atlanta, only a few minutes daily is required to raise vitamin D to appropriately high levels, which are critical for healthy bones and protection against many forms of cancer and multiple sclerosis. In more northern latitudes, the sun is at too low an angle for half the year to provide sufficient UV radiation. Also, many people throughout the U.S. wear sunscreen, which blocks "D" synthesis. That's why I recommend 2,000 IU of supplemental vitamin D daily, no matter where you live.
Andrew Weil, M.D.