Suffocation isn't the only risk. Unlike the effect carbon dioxide has on you, displaces oxygen and is more or less inert, oxygen (O2) is a caustic substance and fire hazard. If your CO2 race is not from a planet with large amounts of oxygen their equipment will need special treatment, their skin may become irritated, begin breaking down in a messy leprous goo, or simply burst into flames.
Like any hazardous environment suit a positive pressure would need to be maintained in the suit. This will keep the potentially toxic outside gases from getting at the skin of mucous membranes of the subject. Additionally, some amount of exo-skeletal support may be required to deal with higher gravity.
Re: OT: Carbon Dioxide races -> known vs unknown -> terraforming mars -> is or is not
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