normal carbon dioxide levels in the air are neglible
you normally exhale carbon dioxide at concentrations of 5% and this usually gets rapidly diluted into the air you breathe UNLESS it is a enclosed space
your body at rest usually produces 0.2L/minute of CO2 and this will be exhaled into the enclosed space resulting in a rapid build up
inhaled concentrations of carbon dioxide at levels over 3% will cause rising CO2 levels in your body and severe headaches when you wake up, levels over 5% may kill you
minutely rate of rise in FiCO2 = (CO2 production in L/minute) / (Volume of the space in litres)
THUS, for one adult inside an average two person tent of volume 2700L which happens to be fully sealed, the rate of rise = 0.2L/min / 2700 = 74ppm per minute, and this means that in only 400 minutes (6.75hrs), the FiCO2 will reach a dangerous level of 3% and 675 minutes (11hrs) to get to dangerous levels of 5%
Obviously if there were two adults exercising inside the tent, the CO2 production would be much higher!
most inner tents are made of uncoated mesh, untreated polyester (unless very tight weaves > 210T) or highly breathable nylon (including silnylon) which have a high enough effective diffusion rate to easily clear the 200mL/min of CO2 produced by a human. The gas simply passes through the weave down its concentration gradient
Air Permeability: standard uncoated tent fabrics typically have an air permeability ranging from 10 to 50mL/sq.cm/sec at standard test pressures.
in contrast, PU coated fabrics have are relatively impermeable to gas and will trap CO2