Chronic urticaria (ordinary urticaria) is defined as the presence of evanescent wheals which persist for greater than six weeks. Some of the more severe chronic cases have lasted more than 20 years. A survey indicated chronic urticaria lasted a year or more in more than 50% of those affected and 20 years or more in 20% of them.Acute and chronic hives are visually indistinguishable. Related conditions
Angioedema
Angioedema is similar to hives, but in angioedema, the swelling occurs in a lower layer of the dermis than in hives, as well as in the subcutis. This swelling can occur around the mouth, eyes, in the throat, in the abdomen, or in other locations. Hives and angioedema sometimes occur together in response to an allergen, and is a concern in severe cases, as angioedema of the throat can be fatal. Vibratory angioedema
This very rare form of angioedema develops in response to contact with vibration. In vibratory angioedema, symptoms develop within two to five minutes after contact with a vibrating object and abate after about an hour. People with this disorder do not experience dermographism or pressure urticaria. Vibratory angioedema is diagnosed by holding a vibrating device such as a laboratory vortex machine against the forearm for four minutes. Speedy swelling of the whole forearm extending into the upper arm is also noted later. The principal treatment is avoidance of vibratory stimulants. Antihistamines have also been proven helpful. Management
The mainstay of therapy for both acute and chronic hives is education, avoiding triggers and using antihistamines.
These three compounds are formed by means of similar chemical processes, and oil that contains one of the three substances is likely to contain the other two. Oil samples that are suspected to have been ingested by people who later developed TOS often contain all three of these contaminants (among other substances), but are most likely to contain OOPAP. However, when these three substances were given to laboratory animals, OOPAP was not acutely toxic, PAP was toxic only after injection, but not after oral administration, and OPAP was toxic only after injection of high doses. Therefore, none of these three substances is thought to cause TOS. Similar results were obtained after administration of fatty acid anilides.Data discrepancies combined with both a high level of secrecy surrounding the huge investigation and the fact that the first cases of the syndrome were located in Madrid (near the U.S. military base in Torrejón de Ardoz) spread the idea of a conspiracy. Several of those affected by TOS claim they never consumed any of the tainted oil products. Furthermore, the tainted oil was primarily sold in low-cost street markets; yet, a considerable percentage of the patients were wealthy. Another theory suggests the toxic reaction was triggered by organophosphate poisoning (e. g., from pesticide residues in tomatoes) and covered up by the Spanish government and the WHO. See also
Eosinophilia-myalgia syndrome
Ginger Jake
Health crisis
The Jungle
List of food contamination incidents

References
External links
WHO Report: Toxic Oil Syndrome - Ten years of progress