Rider dropped out of the tour for using an Astham Spray ??

So-called beta-2 agonists are banned as asthma medication unless athletes have a therapeutic use exemption allowing them to take formoterol, salbutamol, salmeterol and terbutaline when inhaled, according to the World Anti-Doping Agency (WADA).

These treatments work by copying certain effects of naturally-produced chemicals -- adrenaline and noradrenaline -- to open up the airways so that more air reaches the lungs.

"Because some of the medication is on the banned substance list the IOC must be sure that only asthmatic athletes are using asthma medication," said Ken Van Someren, lead physiologist at the English Institute of Sport.

"Although it should be stressed that therapeutic doses of asthma medication will not have any performance-enhancing effect."

The test involves measuring lung function before exercise and then either after 10 minutes of hard exercise or 10 minutes after an athlete takes asthma medication.

A drop of 10 percent or more in lung function was considered a positive test and a signal the athlete needed to see a doctor to get on medication, he said.

At the Los Angeles Olympics in 1984, 1.7 percent of all athletes who competed sought and were granted exemptions to use asthma medication, according to Joseph Cummiskey, a respiratory physician and member of the IOC.

This figure has steadily risen since then and in the 2004 Athens Games 7.4 percent of the athletes were given exemptions, though some were rejected, he said.