There are at least two risk factors associated with using compressed air, when testing the EVAP system and you should do everything possible to avoid the risks.
1. The oxygen introduced into the fuel tank will inevitably exit along with the flammable gasoline fumes, either through a leak source in the EVAP system or through the EVAP system’s canister vent valve. Ignition can take place at those and other locations if static electricity or some other external ignition source is present, such as when the canister vent valve is back-probed closed on vehicles lacking bidirectional communication. We all know there are many other potential sources for ignition in a workshop environment and many of us have heard or read about gasoline fumes catching on fire, creating a very hazardous situation. Using nitrogen reduces the oxygen content of the mixture in the leaking fumes and reduces the chances of the dispersing vapor from ever reaching the Lower Explosive Limit (LEL) of gasoline.
2. Testing the EVAP system of a vehicle using compressed air usually means that you will be adding the equivalent volume contained in several gallons of vapor space. Under many conditions, there will be enough oxygen added to the fuel vapor space that will warrant a safety concern. Filling the fuel vapor space inside the tank with more than 12% of oxygen volume creates a fuel-to-air mixture that will support combustion. In fact, University studies have shown that it takes only one to five minutes of air introduction from any smoke machine to render much of the fuel tank’s vapor space flammable. There are also documented cases where the fuel pumps inside fuel tanks have developed overheated wiring harnesses hot enough to ignite a flammable mixture. So now any vapor leak leaving the fuel tank can become a sort of “fuse” back into a flammable fuel tank mixture.
Using an inert gas, such as nitrogen, is the most sensible, simplest and least expensive insurance you can have to reduce the risks mentioned above.