Dr Nick Riley calculated it like this:
From spirometry measurements (National Lung Health Education Programme), assuming that about 450ml/min of CO2 are expired per person per minute – this means the net CO2 respired (CO2 produced by respiration) is around 400ml/min
1g of CO2 has a volume of 556ml. Therefore 400ml has weight of 0.72g. Therefore in 1 hour the net CO2 released is 0.72 X 60=43.2g
One day is 43.2 X 24=1036g or 1.036kg.
One year is 1.036 X 365= 378.332Kg or 0.378332 metric tonnes
If the global population is 6.5bn this comes to a total weight of 2.459158000Gt.
For comparison the total world emissions of CO2 from human activity (excluding respiration) is around 27Gt. Of which the per capita average emissions are for a each citizen as follows:
USA 5.4 tonnes/person/annum
UK 2.59
China 0.6
India 0.3
This was Taken from BBC http://www.bbc.co.uk/radio4/science/homeplanet_20060117.shtml
I happend to see this interesting calculation and thought of sharing.
