Which of the following best describes the purpose of negative pressure ventilation?

The purpose of negative pressure ventilation is to effectively remove contaminants from a confined space while simultaneously drawing fresh air into that space. This method is essential in confined space operations where the environment may be hazardous due to the presence of toxic gases, fumes, or insufficient oxygen levels. By using negative pressure, the ventilation system creates a zone of lower pressure inside the space compared to the outside atmosphere, leading to the influx of fresh air. This helps in diluting and removing harmful substances, thereby improving the safety and air quality for any personnel present.

In contrast, the other options do not accurately represent the primary function of negative pressure ventilation. Introducing fresh air while keeping contaminants in does not provide the necessary safety measures for the personnel involved. Circulating air without replacing existing air does not address the issue of contaminants and would be ineffective for ensuring a safe environment. Lastly, creating a vacuum for entry operations does not align with the intent of negative pressure ventilation, which focuses on air quality improvement rather than suction or vacuum effects.