By Bob Baker

During the 1960s, the idea that infections were transmitted through the air was a widely accepted concept. A newly published study had traced a transmission of TB out a window in a hospital, down to another floor and through a hall to a second patient. There was much speculation that many infections and illnesses might be transmitted through airborne routes and there was great interest in how we could sterilize the air in order to prevent disease transmission.

Following Infection control research, however, demonstrated that many, if not most, infections were transmitted through person to person contact; largely as a result of poor personal hygiene (hand washing) practices. The US Centers for Disease Control and Prevention (CDC) focused on personal hygiene practices as the preferred method of preventing spread of infection; especially in health care environments. It became accepted wisdom that facilities where health care providers consistently washed their hands between patient contacts enjoyed low levels of infection compared to those where employees were less careful.

SARS Outbreak

The 2003 outbreak of SARS (severe acute respiratory syndrome) virus resulted in a number of cases where researchers and medical practitioners could not explain how the virus was transmitted from one patient to another. Although personal contact was felt to be the primary route of transmission, there were a number of patients where not close contact could be identified. Researchers began to search for other possibilities.

One of these was Dr. Yuguo Li from the University of Hong Kong. On May 25 of this year, I met with Dr. Li and he reviewed his research into the spread of SARS in the Amoy Gardens Housing Estate in Hong Kong. In an article that was published in the New England Journal of Medicine on April 22, 2004, he and his associates presented strong evidence that this outbreak that eventually infected over 300 residents was most likely spread through air currents. The following is an abstract of the article should be of great interest to all of us who care about Indoor Air Quality and its impact on the health and comfort of our clients:

Background

There is uncertainty about the mode of transmission of the severe acute respiratory syndrome (SARS) virus. We analyzed the temporal and spatial distributions of cases in a large community outbreak of SARS in Hong Kong and examined the correlation of these data with the three-dimensional spread of a virus-laden aerosol plume that was modeled using studies of airflow dynamics.

Methods

We determined the distribution of the initial 187 cases of SARS in the Amoy Gardens housing complex in 2003 according to the date of onset and location of residence. We then studied the association between the location (building, floor, and direction the apartment unit faced) and the probability of infection using logistic regression. The spread of the airborne, virus-laden aerosols generated by the index patient was modeled with the use of airflow-dynamics studies, including studies performed with the use of computational fluid-dynamics and multizone modeling.

Results

The curves of the epidemic suggested a common source of the outbreak. All but 5 patients lived in seven buildings (A to G), and the index patient and more than half the other patients with SARS (99 patients) lived in building E. Residents of the floors at the middle and upper levels in building E were at a significantly higher risk than residents on lower floors; this finding is consistent with a rising plume of contaminated warm air in the air shaft generated from a middle-level apartment unit. The risks for the different units matched the virus concentrations predicted with the use of multizone modeling. The distribution of risk in buildings B, C, and D corresponded well with the three-dimensional spread of virus-laden aerosols predicted with the use of computational fluiddynamics modeling.

Conclusions

Airborne spread of the virus appears to explain this large community outbreak of SARS, and future efforts at prevention and control must take into consideration the potential for airborne spread of this virus.

Message to us

While it has been the practice in the US for the past several decades to discount environmental mechanisms for the spread of infection and disease, we should re-evaluate out position in view of this new and important study. We may discover that HVAC system maintenance and general indoor environmental quality are more important to our overall health and quality of life than we had previously suspected.

Bob Baker is a member of IAQA, ASHRAE, CSPA and Chairman and CEO of BBJ Environmental Solutions, Inc., "The Standard of Care for Indoor Air". BBJ has offices in Tampa and Hong Kong and Mr. Baker follows indoor air quality developments throughout the world. For additional information, Mr. Baker can be reached at (800) 889-2251 or through the company web site at www.bbjenviro.com.