In which toxic atmosphere must employees not be allowed to work?

The correct answer is oxygen deficient environments. In such environments, the concentration of oxygen is below safe levels, typically defined as less than 19.5% oxygen in the air. When workers are exposed to an oxygen deficient atmosphere, it poses serious and immediate health risks, including impaired cognitive function, loss of consciousness, and potentially fatal outcomes if not corrected.

OSHA standards require that workers be trained to recognize the dangers associated with oxygen deficiency and that precautions be taken to ensure safe working conditions. This includes continuous monitoring of oxygen levels and ensuring proper ventilation or the use of respiratory protection if necessary.

Other options such as oxygen-rich environments can be dangerous but are not as critical as those with deficient oxygen, high humidity does not directly impair oxygen levels, and low noise doesn't present a toxic atmosphere, thus they are not applicable in the same context.