The link will bring you to research placing the average output of carbon per breath at .010 grams. Simple multiplication at 6 breaths per minute, times 60 minutes, times 24 hours, times 365 days = 31,536grams (69.5 pounds) of carbon. Multiply this by the US population (http://www.census.gov/main/www/popclock.html): 301,099,804 and then divide by 2000# and you get the 10 million ton figure.
The entire world population exhales approximately 228 million tons of carbon yearly based on these calculations. It would be great if someone could corroborate or refute this.