Which of the following is NOT allowed with re-circulating systems in dangerous environments?

Using room air in re-circulating systems within dangerous environments is not permitted due to safety concerns associated with the potential accumulation of hazardous gases or fumes. In environments where there are flammable materials, toxic substances, or other risks, relying on indoor air for circulation can lead to a buildup of harmful air contaminants. This could compromise air quality and pose a health and safety hazard.

In contrast, utilizing outside air helps ensure that fresh, uncontaminated air is circulated, which can dilute and dissipate any hazardous vapors or gases present in an environment. Variable speed integration allows for better control over airflow, which can enhance safety by adapting to changes in environmental conditions. Adequate ventilation is crucial in maintaining a safe atmosphere, allowing for the dispersal of hazardous substances as well. All these factors contribute to maintaining safety standards in environments that may pose risks.