At 40,000 feet, WHAT ambient air?

If there was ambient air, you would not need need a mask.

Am I missing some Gas Law of Physics here?

Hot Air Beef: The safety video on US carriers uses some sugar coated PC language like "In the event of a change in cabin pressure, a mask yada yada yada."

Don't want to scare grandma by letting her know that planes are made by people, are not perfect, can and do BREAK sometimes, least of all during the Safety Briefing for Her Own Good.

My compliments for straight talking truth to Singapore Airlines who recently told me "In the event we HAVE A LOSS OF CABIN PRESSURE. . ."

Thanks SQ for having the guts to say what actually happens.