What should be done first if a diver has joint pain due to altitude DCS that resolves before reaching one ATA?

Providing 100 percent oxygen breathing for a duration of 2 hours is the most appropriate first step in this scenario. When a diver experiences joint pain from Decompression Sickness (DCS)—even if the symptoms resolve before reaching one ATA—the administration of pure oxygen can significantly aid in reducing the residual nitrogen bubbles that may still exist in the body.

By breathing 100 percent oxygen, the diver increases the partial pressure of oxygen in their system, which enhances the elimination of nitrogen from the tissues. This can help alleviate symptoms and prevent the worsening of the condition. Oxygen therapy is a critical component of the treatment protocol for DCS and can be employed while preparing for further medical evaluation.

Transporting the diver to a medical facility is important, but immediate oxygen treatment should take precedence to provide first aid for the symptoms experienced. Recompressing the diver may not be necessary immediately if the pain has resolved, and monitoring the diver without treatment could risk the condition worsening. Thus, administering oxygen helps stabilize the diver's condition effectively while arrangements for further medical care are made.