The human activities which would lead to an increase in CO2 content of air are:
(i) Respiration is the natural way to release of CO2 by both plants and animals. It is balanced by the release of oxygen by plants. So, it is not harmful for the environment.
(ii) Deforestation increases the level of CO2 in the environment. Trees carry out photosynthesis and convert CO2 into organic compounds such as glucose, starch, etc. In their absence, CO2 cannot be utilized.
(iii) Combustion of fuels leads to increase in CO level in the atmosphere. Fuels are burnt to carry out activities like cooking, transportation and in industrial processes.