What is the characteristic of carbon dioxide (CO2) that impacts human safety?

The characteristic of carbon dioxide (CO2) that significantly impacts human safety is its ability to deplete oxygen levels in the environment. When CO2 concentrations rise, they can displace oxygen in the air, leading to a reduction in the amount of breathable air available. This is particularly dangerous in enclosed or poorly ventilated spaces, where high levels of CO2 can accumulate. As the oxygen level decreases, individuals may experience symptoms ranging from mild headaches and dizziness to severe hypoxia, which can be life-threatening. Hence, the effect of CO2 on oxygen levels is crucial for understanding its safety implications for humans.

Other options, while relevant to the properties of CO2, do not directly address human safety in the same impactful way. For instance, CO2 being a greenhouse gas relates more to environmental concerns than immediate human health risks. The aspects of it being combustible or having no odor also do not pose direct threats that are as critical as its potential to displace oxygen.