Hospital engineers should air on the side of caution

NIOSH urges preventing public access to intakes

Hospital engineers and safety officials should take measures to prevent public access to outdoor air intakes to make health care facilities a less attractive target for bioterrorism, public health officials recommend.

"You could see why someone might want to target a hospital — just to reduce your capability to respond to whatever other event may be created if there was a two-stage attack," says Kenneth R. Mead, MS, PE, a research mechanical engineer at the National Institute for Occupational Safety and Health (NIOSH) in Cincinnati. "If the goal of the terrorist was to strike fear and disrupt the community, that certainly would be [one facility] they could go after."

A branch of the Atlanta-based Centers for Disease Control and Prevention (CDC), NIOSH is the principal agency behind a recent report on securing public buildings to prevent bioterrorism attacks. In November 2001, following the discovery that letters containing Bacillus anthracis had been mailed to targeted locations in the United States, the Secretary of the U.S. Department of Health and Human Services requested site assessments of an array of public- and private-sector buildings by a team of engineers and scientists from NIOSH and CDC. In November 2001, the team assessed six buildings, including a large hospital and medical research facility, a museum, a transportation building, two large office buildings, and an office/laboratory building. In January 2002, more building assessments were conducted at CDC campuses in Atlanta and, in April 2002, at a large, urban transportation facility. A total of 59 buildings were evaluated during this five-month period.1

One of the prime vulnerabilities of hospitals and other institutional buildings is access to the air intake, giving bioterrorists a possible way to introduce an agent into the building ventilation system. The report states that "publicly accessible outdoor air intakes located at or below ground level are at most risk." That is due to their accessibility, ease of identification, and because most chemical or biological agents released near a building may remain close to ground level. "Securing the outdoor air intakes is a critical line of defense," the report states.

Relocating accessible air intakes to a publicly inaccessible location is preferable. Ideally, the intake should be located on a secure roof or high sidewall, NIOSH recommends. The lowest edge of the outdoor air intakes should be placed at the highest feasible level above the ground or above any nearby accessible level (i.e., adjacent retaining walls, loading docks, handrail). These measures are also beneficial in limiting the inadvertent introduction of other types of contaminants, such as landscaping chemicals, into the building.

"In terms of new construction, people are starting to think about this [air intake access] as a new issue, whereas in the past they wouldn’t have thought it was a problem," Mead says. "Interestingly, people involved in the indoor air quality arena have been trying to get the air intakes up off the ground anyway, especially those that are below ground and tend to be done in window wells. There are a lot of reasons -— terrorism aside — why you would not want a ground-level intake."

If relocation of outdoor air intakes is not feasible, intake extensions can be constructed without creating adverse effects on heating-ventilation-air conditioning (HVAC) performance. Depending upon budget, time, or the perceived threat, the intake extensions may be temporary or constructed in a permanent, architecturally compatible design, NIOSH recommends. "The goal is to minimize public accessibility," the report states. "In general, this means the higher the extensions, the better — as long as other design constraints (excessive pressure loss, dynamic and static loads on structure) are appropriately considered." In addition to the air intake recommendations, NIOSH warned against doing any of the following:

1. Do not permanently seal outdoor air intakes. Buildings require a steady supply of outdoor air appropriate to their occupancy and function. This supply should be maintained during normal building operations. Closing off the outdoor air supply vents will adversely affect the building occupants and likely result in a decrease in indoor environmental quality and an increase in indoor environmental quality complaints.

2. Do not modify the HVAC system without first understanding the effects on the building systems or the occupants. This caution directly relates to the recommendation that building owners and managers should understand the operation of their building systems. If there is uncertainty about the effects of a proposed modification, a qualified professional should be consulted.

3. Do not interfere with fire protection and life safety systems. These systems provide protection in the event of fire or other types of events. They should not be altered without guidance from a professional specifically qualified in fire protection and life safety systems.

(Editor’s note: The complete NIOSH report is available at


1. Centers for Disease Control and Prevention. Notice to readers: Protecting building environments from airborne chemical, biologic, or radiologic attacks. MMWR 2002; 51:789.