With a high concentration of ppm (in a sealed environment, you can reach thousands) you can directly paralyze the respiratory center in the brainstem, thus blocking signals to the muscles that control breathing. At the same time, you inhibit mitochondrial cytochrome oxidase, which blocks the use of oxygen by cells. Essentially, you cause chemical hypoxia that immediately affects neurons. These two mechanisms combined are triggered after a few breaths and render the brain incapable of receiving oxygen and activating respiratory commands, leading to neurological collapse. Olfactory paralysis occurs, in some people, as early as 150-200 ppm. If you've thoroughly saturated the environment, you can't perceive anything, not even smell.