Just as in the anhydrous method, Meth cooks using a Red P lab are extracting the ephedrine or pseudoephedrine from common medications. The difference is in using red phosphorus as the catalyst - the chemical to speed up the reaction process and makes it go to completion.
Cooks get the chemical from various sources; the most common are the strike plates from match books or boxes and from flares.
Both methods share many of the same hazards to the cooks, their families and unsuspecting neighbors or first responders. But Red P labs pose a few unique dangers. Red phosphorus may produce phosphine gas, an extremely poisonous and flammable gas.
It is highly toxic and may be fatal if absorbed or inhaled.
If you notice someone buying an excessive amount of matches or flares, or if you see piles of these items discarded, be extremely careful. You may notice other signs of Meth labs that are common to every method, such as large quantities of the other ingredients and the equipment used to cook the Meth. It is highly recommended that people without the specialized gear and training not approach any type of Meth lab. Call your local law enforcement to investigate, explaining why you think it may be a Red P Meth lab.
Information from Drug Enforcement Agency, National Drug Intelligence Center