Is too much CO2 in the air toxic?
CO2 forms carbonic acid in water, which helps to dissolve rocks and mountains in envitonmental science. It would slightly acidify your body with what is called a weak acid.

5% can cause acidosis and loss of conciousness, without lack of oxygen.

3% would devastate most chalk based sealife, corals and shells. 

Perhaps lifelong breathing of 3% co2 would have serious effect on alkaline processes in the body, osteoporosis, and would have to be counterbalanced by the body to maintain its blood ph above 7.35, with alkaline chemicals like sodium. 

Also check out this fact sheet.