Control of Infectious Aerosols

In today’s world, ensuring the health and well-being of building occupants has become a top priority. One essential aspect of achieving this is maintaining optimal indoor air quality (IAQ) through clean and efficient ventilation systems. ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) Standard 241, “Control of Infectious Aerosols establishes the minimum requirements to reduce the risk of airborne disease transmission in buildings. This standard applies to both new and existing buildings. It provides requirements for HVAC system design, installation, operation, and maintenance.

While ASHRAE 241 is a comprehensive engineering document, the standard has four main topics that address the control of infectious aerosols. They are Infection Risk Management Mode, Requirements for Equivalent Clean Airflow Rate, Use of Filtration and Air Cleaning Technology, and Planning and Commissioning.

Control of Infectious Aerosols with HVAC

Of the four important topics in ASHRAE 241, two have a strong focus on the HVAC System. Per ASHRAE:

  • Requirements for Equivalent Clean Airflow Rate – Sets requirements for equivalent clean airflow rate target (in liters per second) per occupant of pathogen free air flow, reducing the risk of infection.
  • Planning and Commissioning – Provides assessment and planning requirements for being ready for the times when there is an event with increased disease-causing pathogen transmissions. The standard has a building readiness plan, that documents procedures for assessing existing or new HVAC systems to determine if they are working properly and attributing to the equivalent clean air delivered to spaces.

Ventilation Rates and Air Changes per Hour (ACH)

Ventilation rates in every building other than healthcare facilities were never designed for the control of infectious aerosols. During the COVID-19 pandemic, organizations upgraded their HVAC systems and filtration without consideration for how much increased ventilation was required. ASHRAE 241 now addresses this in a very detailed manner. See this July 2023 article from It’s Airborne for an easy-to-understand breakdown.

However, the Centers for Disease Control (CDC) in a May 2023 update distilled this harrowing data into an average of 5 (or more) ACH for most normally occupied buildings.

HVAC Assessments and Maintenance

Both ASHRAE and the CDC emphasize the critical importance of assessing and maintaining the HVAC system for the control of infectious aerosols in the built environment. In fact, ASHRAE 62.1, “Ventilation and Acceptable Indoor Air Quality” dedicates section 8 to outline HVAC operations and maintenance. Both ASHRAE standards outline monthly, quarterly, and annual assessments of specific HVAC components.

Building upon the ASHRAE guidance, The CDC makes two clear appeals on their Ventilations in Buildings webpage linked above.

Section 4 under “Improving Air Circulation” recommends to “Rebalance or adjust HVAC systems to increase total airflow to occupied spaces when possible”. Then section 2 under “Improving Air Cleanliness” goes further calling for the Inspection of HVAC Systems to:

  • Ensure ventilation systems operate properly and are up to date on maintenance.
  • Make sure air filters are properly sized and within their recommended service life.
  • Inspect filter housing and racks to ensure appropriate filter fit and minimize air that flows around, instead of through, the filter.

How Pure Air Helps

WTI Pure Air helps with ASHRAE- and CDC compliance in three ways. We test, clean, and restore HVAC systems from an engineering and environmental focus!

HVAC Conditions Assessment

We have three tiers or types of HVAC assessments. First, is a visual inspection of conditions with type of equipment, photo documentation, and dimensions. Second, environmental samples are taken from the air handler unit and laboratory analysis is provided. Finally, an engineering assessment is conducted to determine operational performance. These tiers can be combined depending on the scope or intent of the survey. However, in the case of the control of infectious aerosols, the engineering assessment will provide the most valuable information on airflow, static pressure, and thermal capacity to properly calculate and maintain ventilation. Based on the findings of the HVAC assessment, recommendations are made for the cleaning and restoration of the system.

HVAC Steam Cleaning

Fouled evaporator coils in air handler units obstruct airflow. It’s as simple as that. High temperature, low pressure steam cleaning of the coils and interior components not only restore airflow, but also disinfect the AHU!

HVAC Restoration

Once the coils and AHU have been thoroughly cleaned it can be refinished with high-performance coatings. These coatings are designed and certified to be applied within HVAC equipment. They provide anti-microbial and water resistance. Additionally, fiberglass free, hydrophobic insulation can be installed. Finally, the old blower and motor can be upgraded to an electronically controlled motor fan array. A fan array is just as it sounds, a wall of multiple fans designed to provide redundancy. In an old blower system if the blower, motor, or belt breaks, you lose airflow from the AHU until it can be repaired. With a fan array, if one of the fans fails, the rest make up for the lost fan and airflow is never disrupted. This is critical to continuous airflow and ventilation!

Environmental Duct Cleaning

Over time, the ductwork in a building can get dirty with dust and debris. It is not only important to keep the ducts clean, but also keep the equipment located within it free from airflow obstructions. Often equipment like VAV terminals and reheat coils are overlooked in commercial duct cleaning. That’s why it is crucial to include inline equipment in the duct cleaning scope so optimized ventilation is maintained.

ASHRAE Standard 241: Control of Infectious Aerosols and the CDC’s recommendation for a minimum of five air changes per hour is pivotal in optimizing indoor air quality and ensuring occupant well-being. Clean HVAC and air distribution systems are integral to achieving this ventilation target. By adhering to these standards and maintaining a commitment to clean air systems, we can create indoor environments that are conducive to health, comfort, and productivity. Contact WTI Pure Air to get started today!