Cheilitis is a medical condition characterized by inflammation of the lips. The inflammation may include the perioral skin (the skin around the mouth), the vermilion border, or the labial mucosa. The skin and the vermilion border are more commonly involved, as the mucosa is less affected by inflammatory and allergic reactions.Cheilitis is a general term, and there are many recognized types and different causes. According to its onset and course, cheilitis can be either acute or chronic. Most cheilitis is caused by exogenous factors such as dryness (chapping) and acute sun exposure. Allergic tests may identify allergens that cause cheilitis. Chapped lips
Chapped lips (also known as cheilitis simplex or common cheilitis) is characterized by the cracking, fissuring, and peeling of the skin of the lips, and is one of the most common types of cheilitis. While both lips may be affected, the lower lip is the most common site. There may also be burning or the formation of large, painful cracks when the lips are stretched. Chronic cheilitis simplex can progress to crusting and bleeding. Counterintuitively, constant licking of the lips causes drying and irritation, and eventually the mucosa splits or cracks. The lips have a greater tendency to dry out in cold, dry weather. Digestive enzymes present in the saliva may also irritate the lips, and the evaporation of the water in saliva saps moisture from them.
The latter is obtained by the treatment of magnesium chloride MgCl2 solutions, typically seawater, with limewater or milk of lime. Mg2+  + Ca(OH)2  →  Mg(OH)2  +  Ca2+Calcining at different temperatures produces magnesium oxide of different reactivity. High temperatures 1500 – 2000 °C diminish the available surface area and produces dead-burned (often called dead burnt) magnesia, an unreactive form used as a refractory. Calcining temperatures 1000 – 1500 °C produce hard-burned magnesia, which has limited reactivity and calcining at lower temperature, (700–1000 °C) produces light-burned magnesia, a reactive form, also known as caustic calcined magnesia. Although some decomposition of the carbonate to oxide occurs at temperatures below 700 °C, the resulting materials appears to reabsorb carbon dioxide from the air. Applications
Heating elements
MgO is prized as a refractory material, i.e. a solid that is physically and chemically stable at high temperatures. It has two useful attributes: high thermal conductivity and low electrical conductivity. Filling the spiral Calrod range top heating elements on kitchen electric stoves is a major use. "By far the largest consumer of magnesia worldwide is the refractory industry, which consumed about 56% of the magnesia in the United States in 2004, the remaining 44% being used in agricultural, chemical, construction, environmental, and other industrial applications." MgO is used as a basic refractory material for crucibles. Fireproofing
It is a principal fireproofing ingredient in construction materials. As a construction material, magnesium oxide wallboards have several attractive characteristics: fire resistance, termite resistance, moisture resistance, mold and mildew resistance, and strength.