What must be done to a dressing before covering a white phosphorus burn?

When dealing with a white phosphorus burn, it is crucial to cover the affected area with a wet dressing. White phosphorus can continue to burn unless it is thoroughly cooled and removed from the skin. A wet dressing helps to extinguish the burning material by cooling it down and preventing further injury. The moisture in the dressing also assists in easing the pain associated with the burn and helps to keep the burn area clean and free from debris that might lead to infection.

The application of a wet dressing is a vital step in treating these types of burns due to the active nature of white phosphorus, as opposed to simply covering it with a dry dressing or other materials that do not provide the necessary cooling effect. This approach is a standard practice in first aid for such burns, ensuring a better outcome for the affected individual.