Because exhaled air contains VOCs.
The gas exhaled is 4% to 5% by volume of carbon dioxide, about a 100-fold increase over the inhaled amount. The volume of oxygen is reduced by a small amount, 4% to 5%, compared to the oxygen inhaled.
The typical composition is:
5.0–6.3% water vapor
74.4% nitrogen
13.6–16.0% oxygen
4.0–5.3% carbon dioxide
1% argon and several parts per million (ppm) of hydrogen and carbon monoxide, 1 ppm of ammonia and less than 1 ppm of acetone, methanol, ethanol and other volatile organic compounds