It's not waxed cotton; it's chemically waterproofed. I am a proponent of having things of this sort done professionally, not least of which because of the health hazards already mentioned. I am also told that the spray products may unevenly change the color of the fabric.
I have owned a pure cotton raincoat from J Press (actually made by Grenfell) for several years, and the waterproof treatment has faded substantially. My local dry cleaner informed me that they can't refresh the treatment.
Is there a mail order service worthy of recommendation?