Unsafe at Any Air Change Rate: The Myth of the Well-Mixed Space

Thomas Smith, 3Flow

People working in laboratories and critical workspaces are potentially exposed to airborne hazards. They rely on proper design and operation of the lab ventilation systems to keep them safe. Air changes per hour (ACH) is commonly specified to indicate the quantity of air required for safety, but there is little guidance on how to select an appropriate rate and then determine whether the air change rate provides effective dilution and removal of airborne contaminants.

A risk-based, systems approach can be applied to evaluate hazardous activities, associate with a level of risk, and recommend appropriate air change rates. However, specification and application of air change rates assumes that the space of concern is well-mixed, meaning contaminants are diluted through disbursement and homogeneous distribution of concentrations throughout the volume of the room. Results of air tracer tests to evaluate ventilation effectiveness demonstrate that many spaces are not well-mixed and specification of air change rates without measurement of ventilation effectiveness can lead to a false sense of safety.

This paper will describe why airflow patterns through a space are more important to air quality and occupant safety than the number of air changes per hour.

Learning Objectives

  • Describe a risk-based, systems approach to design and operation of lab ventilation systems;
  • Describe how risk can be evaluated and used to assign airflow specifications;
  • Describe why airflow patterns are more important to safety than the number of air changes per hour; and
  • Explain how air tracer tests can be used to evaluate ventilation effectiveness and the factors affecting the well-mixed assumption.


Tom Smith is the President and CEO of 3Flow. Mr. Smith has worked for more than 30 years helping facilities provide safe, efficient and sustainable labs. He holds degrees from NCSU and UNC in Engineering and Industrial Hygiene. He has served on numerous standards committees for ventilation and is a member of the I2SL Board of Directors.


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