"Modified atmosphere packaging systems" employ carbon monoxide to keep beef, pork and fish looking good on store shelves. While CO is harmful if inhaled, it's "generally recognized as safe" for use in packaging by the FDA.
In layman's terms, CO interacts with molecules in meat to enhance redness and reduce brownness. Though accepted in the U.S., it's somewhat controversial, since the technique can keep meat looking fresher than it actually is. The practice is banned in Canada, Japan, Singapore and the EU.