There are two things about Oxygen free environments. First, humans can't actually detect Oxygen. We can detect CO2, largely because the build up in our blood m

There are two things about Oxygen free environments. First, humans can't actually detect Oxygen. We can detect CO2, largely because the build up in our blood makes it more acidic, but we can't actually tell how much Oxygen is in the air or in our bodies. The negative feeling of holding your breath isn't caused by insufficient air, but the buildup of CO2 in your blood due to metabolism. As such, humans actually can't detect an oxygen free room, as long as it isn't full of CO2 (we'd notice that). Second, part of how we absorb gas is via the difference in gas pressure between our blood and the air. When the partial pressure of CO2 in our blood is higher than the air, it diffuses out, and when the partial pressure of O2 in the air is higher than our blood it tends to dissolve into solution. In an Oxygen free environment any O2 in our blood not already bound up by Hemoglobin rapidly diffuses away, robbing us of O2 faster than holding our breath and cell metabolism normally would, resulting in loss of consciousness very quickly. The combination of these is why it is advised you never enter an Oxygen free environment without a self contained breathing apparatus. Industrial reports are littered with tragic stories of multiple people entering an N2 or Halogen environment to rescue their friends, only to perish themselves.