I believe the US OSHA limit is 20ppm for an 8hr shift? That might not be right either
Being UK based I'm more familiar with the HSE guidance which gives a dosing formula based in exposure time (CO being a cumulative poison).
Typically I use a pessimistic figure of 450ppm to consider an area impaired (I work in offshore oil and gas major accident safety). That won't kill you but you won't be able to hang around for too long.
i just checked my green book again and it's 50PPM in an 8hr shift.