Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Exposure

This article discusses the issues associated with protecting your building from the risk of airborne chemical, biological & radiological (CBR) exposure. It is intended to provide guidance regarding measures that may be taken to prepare for a potential CBR risk, rather than in response to an actual CBR event.

Overview

Properly designed, installed, and maintained air-filtration and air cleaning systems can reduce the effects of a CBR agent release, either outside or within a building, by removing the contaminants from the building’s air supply. You who are building owners, managers, designers, and maintenance personnel need reliable information about filtration and air-cleaning options. You need to know (1) what types of air-filtration and air-cleaning systems are effective for various CBR agents, (2) what types of air-filtration and air-cleaning systems can be implemented in an existing HVAC system, (3) what types of air-filtration and air-cleaning systems can be incorporated into existing buildings when they undergo comprehensive renovation, and (4) how to properly maintain the air-filtration and air-cleaning systems installed in your building. Proper air filtration and air cleaning, combined with other protective measures documented and referenced in the National Institute for Occupational Safety and Health (NIOSH) Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks and elsewhere, can reduce the risk and mitigate the consequences of a CBR exposure.

Measures outlined in this article also provide the side benefits of improved HVAC efficiency: increased building cleanliness, limited effects from accidental releases, and generally improved indoor-air quality. These measures may also prevent cases of respiratory infection and reduce exacerbations of asthma and allergies among building occupants. Together, these accrued benefits may improve your workforce productivity.

Considerations before increasing filtration

In addition to proper filter or sorbent selection, several issues must be considered before installing or upgrading filtration systems:

·       Filter bypass is a common problem found in many HVAC filtration systems. Filter bypass occurs when air—rather than moving through the filter—goes around it, decreasing collection efficiency and defeating the intended purpose of the filtration system. Filter bypass is often caused by poorly fitting filters, poor sealing of filters in their framing systems, missing filter panels, or leaks and openings in the air-handling unit between the filter bank and blower. By simply improving filter efficiency without addressing filter bypass, you provide little if any benefit.

·       Cost is another issue affected by HVAC filtration systems. Lifecycle cost should be considered (initial installation, replacement, operating, maintenance, etc.). Not only are higher-efficiency filters and sorbent filters more expensive than the commonly used HVAC system filters but also fan units may need to be changed to handle the increased pressure drop associated with the upgraded filtration systems. Although improved filtration will normally come at a higher cost, you can partially offset many of these costs by the accrued benefits, such as cleaner and more efficient HVAC components and improved indoor environmental quality.

·       The envelope of your building matters. Filtration and air cleaning affect only the air that passes through the filtration and air-cleaning device, whether it is outdoor air, re-circulated air, or a mixture of the two. Outside building walls in residential, commercial, and institutional buildings are quite leaky, and the effect from negative indoor air pressures (relative to the outdoors) allows significant quantities of unfiltered air to infiltrate the building envelope. Field studies have shown that, unless specific measures are taken to reduce infiltration, as much air may enter a building through infiltration (unfiltered) as through the mechanical ventilation (filtered) system. Therefore, you cannot expect filtration alone to protect your building from an outdoor CBR release. This is particularly so for systems in which no make-up air or inadequate overpressure is present. Instead, you must consider air filtration in combination with other steps, such as building pressurization and envelope air tightness, to increase the likelihood that the air entering the building actually passes through the filtration and air-cleaning systems.

CBR agents may travel in the air as a within a gas particulate. A diagram of the relative sizes of common air contaminants (e.g., tobacco smoke, pollen, dust) is shown below. CBR agents could potentially enter a building through either an internal or external release.

The Bottom Line

Simply stated, filtration and air cleaning remove unwanted material from an air stream. For HVAC applications, this involves air filtration and, in some cases, air cleaning (for gas and vapor removal). The collection mechanisms for particulate filtration and air-cleaning systems are very different.

A higher MERV indicates a more efficient filter at capturing particles. As mechanical filters load with particles over time, their collection efficiency increases essentially reducing your risk of exposure.

A more detailed discussion of air-filtration principles can be found in the National Air Filtration Association’s (NAFA) Guide to Air Filtration [NAFA 2001a] and the ASHRAE Handbook: HVAC Systems and Equipment [ASHRAE 2000].

The formation in this taken from the Department of Health and Human Services Centers for Disease Control and Prevention National Institute for Occupational Safety and Health. This information is not conclusive.