Engineering Controls Database
Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments
|Terrorism events have increased interest in the vulnerability of U.S. workplaces, schools, and other occupied buildings to chemical, biological, or radiological (CBR) threats.|
|Of particular concern are a building’s heating, ventilating, and air-conditioning (HVAC) systems, as they can become entry points and distribution systems for many hazardous contaminants, including CBR agents.|
The information in this data-base write-up is taken from the following document: Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments [NIOSH 2003]. It is intended as a companion document of to a previously released NIOSH document titled Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks [NIOSH 2002]. This newer NIOSH document discusses air-filtration and air-cleaning issues associated with protecting building environments from a CBR attack. It provides information about issues that should be considered when assessing, installing, and upgrading filtration systems—along with the types of threats that can be addressed by air-filtration and air-cleaning systems. It is intended to provide guidance regarding measures that may be taken to prepare for a potential CBR attack, rather than in response to an actual CBR event. The complex issues regarding response and cleanup in the aftermaths of an actual CBR event are situation dependent and are beyond the scope of this guidance document.
Filtration and air-cleaning systems may protect a building and its occupants from the effects of a CBR attack. Although it is impossible to completely eliminate the risk from an attack, filtration and air cleaning systems are important components of a comprehensive plan to reduce the consequences. CBR agents can effectively be removed by properly designed, installed, and well-maintained filtration and air-cleaning systems. These systems have other benefits besides reducing clean-up costs and delays, should a CBR event occur. These benefits include improving building cleanliness, improving HVAC system efficiency, potentially preventing cases of respiratory infection, reducing exacerbations of asthma and allergies, and generally improving building indoor air quality. Poor indoor air quality has also been associated with eye, nose, and throat irritation, headaches, dizziness, difficulty concentrating, and fatigue [Spengler et al. 2000].
The following is a brief summary of the information provided in the NIOSH guidance document [NIOSH 2003]. Detailed information can be found in that document.
Filtration and air-cleaning principles
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).
• Particulate air filtration: Particulate air filters are classified as either mechanical filters or electrostatic filters (electrostatically enhanced filters). Although there are many important performance differences between the two types of filters, both are fibrous media and used extensively in HVAC systems to remove particles, including biological materials, from the air.
• Gas-phase air cleaning: Some HVAC systems may be equipped with sorbent filters, designed to remove pollutant gases and vapors from the building environment. Sorbents use one of two mechanisms for capturing and controlling gas-phase air contaminants—physical adsorption and chemisorptions. Both capture mechanisms remove specific types of gas-phase contaminants from indoor air.
Recommendations regarding filter and sorbent selection, operations, upgrade, and maintenance
Before selecting a filtration and air-cleaning strategy that includes a potential upgrade in response to perceived types of threats, the building owner or manager should develop an understanding of the building and its HVAC system. A vital part of this effort will be to evaluate the system thoroughly. Assess how the HVAC system is designed and intended to operate and compare that to how it currently operates. In large buildings, this evaluation is likely to involve many different air-handling units and system components.
• Particulate filter selection, installation, use, and upgrades
o Consider system performance, filter efficiencies, and particle size of interest.
o Consider all of the elements affected by filter upgrades.
o Conduct periodic quantitative performance evaluations.
• Sorbent selection, installation, and use
o Understanding sorbent properties and their limitations.
o Understand performance parameters and prevent breakthrough.
o Establish effective maintenance schedules on predicted service life.
o Don’t reuse chemically active sorbents.
• Filter or sorbent bypass and air filtration
Ideally, all airflow should pass through the installed filters of the HVAC system. However, filter bypass, a common problem, occurs when air flows around a filter or through some other unintended path. Preventing filter bypass becomes more important as filter collection efficiency and pressure drop increase. Airflow around the filters result from various imperfections, e.g., poorly sealed filters, which permit particles to bypass the filters, rather than passing directly into the filter media. Filters can be held in place with a clamping mechanism, but this method may not provide an airtight seal. The best high-efficiency filtration systems have gaskets and clamps that provide an airtight seal.
• Recommendations regarding operations and maintenance
o Do not attempt HVAC system maintenance following a CBR release without first consulting appropriate emergency response and/or health and safety professionals.
o Understand how filter type affects change-out schedules.
o Ensure maintenance personnel are well trained.
o Handle filters with care and inspect for damage.
o Wear appropriate personal protective equipment when performing change-out.
• Note on Emerging technologies
Recently, a number of new technologies have been developed to enhance or augment HVAC filtration systems. Many of these technologies have taken novel approaches to removing contaminants from the building air stream. While some of these new systems may be highly effective, many are unproven. Before committing to one of these new technologies for the protection of a building and its occupants, the vendor should be required to provide evidence that demonstrates the effectiveness for the relevant application.
Costs associated with air filtration and air-cleaning systems can be divided into three general categories: initial costs, operating costs, and replacement costs. Although some users might consider only the initial costs when selecting an appropriate filtration system, it is important to weigh carefully all of the life-cycle costs. The HVAC design engineer should assist you in understanding the costs and benefits of various air-filtration options.
• Initial costs.
• Operating costs.
• Replacement costs.
• Cost data.
Appendix A: Occupational health and safety (OHS) building air protection members.
Appendix B: CBR threats
The effects of the various CBR agents can vary widely. A brief description
of the effects of the different classes of agents is provided in this appendix.
• Classical chemical warfare agents.
• Toxic industrial chemicals and materials.
• Biological agents
• Radiological hazards
Appendix C: Gas-phase air-cleaning principles
Sorbents capture gas-phase air contaminants by physical adsorption or chemisorption. Physical adsorption results from the electrostatic interaction between a molecule of gas or vapor and a surface. Solid adsorbents—such as activated carbon, silica gel, activated alumina, zeolites, porous clay minerals, and molecular sieves—are useful because of their large internal surface area, stability, and low cost. Many of these sorbents can be regenerated by application of heat or
• Chemisorption, adsorption, and breakthrough concentrations.
• Types of sorbent materials.
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NIOSH . Guidance for: Protecting building environments from airborne chemical, biological, or radiological attacks. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication Number 2002-139.
NIOSH . Guidance for: Filtration and air-cleaning systems to protect building environments. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication Number 2003-136.
Spengler JD, Samet JM, McCarthy JF . Indoor air quality handbook. New York, NY: McGraw-Hill.