What distinguishes exhaust ventilation systems from balanced ventilation systems?

Exhaust ventilation systems are characterized by their method of operation, which primarily involves the removal of indoor air without simultaneously providing a direct mechanism for bringing in outside air. This one-way process creates a slight negative pressure inside the building, allowing fresh air to enter through any available leakage points or passive vents. This is distinct from balanced ventilation systems, which actively bring in and simultaneously exhaust air, ensuring that the volume of air going out matches the volume coming in.

While other options touch on characteristics of various ventilation systems, they do not accurately define the unique function of exhaust ventilation systems. For example, natural airflow isn't exclusive to exhaust systems, as some balanced systems may also utilize natural methods. The operational schedule can be a feature of any ventilation system and doesn't specifically apply to exhaust ventilation. Lastly, while mechanical assistance is commonly used in ventilation systems, exhaust ventilation systems do not inherently require it for all air changes; they can operate based purely on passive air movement created by the exhaust process. Therefore, the defining characteristic highlighted in the correct answer is what sets exhaust systems apart from balanced ventilation systems.