Led by Washington University in St. Louis and involving the University of Colorado at Boulder and San Diego State University, the new study assessed the microbe population in a warm hospital therapy pool through a ribosomal RNA analysis that involved cloning and sequencing genes. The team concluded that the microbes, many of them pathogenic, were enriched just inches above the pool's surface by as much as 60 percent.
The microbes included "Mycobacterium avium," which can cause "lifeguard lung" and which was present in nine ill lifeguards who worked at the facility.
The technique used in the study is 1,000 times more sensitive than standard microbe-culturing techniques used at most indoor pool facilities today, the researchers said. The two-year study involved a molecular survey of a common RNA gene found in all life forms, eventually matching some cloned and sequenced bacteria to the "Mycobacterium avium" bacterium present in pool attendants ill with hypersensitivity pneumonitis, a lung condition that mimics pneumonia symptoms.
The study was published in the March 14 issue of the Proceedings of the National Academy of Sciences. The paper was authored by Lars Angenent of Washington University; Norman Pace, Mark Hernandez and Allison Amand of CU-Boulder; and Scott Kelley of San Diego State University.
The findings were somewhat surprising," said Pace, a professor of molecular, cellular and developmental biology at CU-Boulder. "People should be very aware of the risks in using indoor pools or hot tubs."
Washington University's Angenent, a former postdoctoral researcher at CU-Boulder, said the high presence of aerosol pathogens poses the greatest concern for very young children, the elderly and people whose immune systems are compromised. The pool attendants probably became ill because they were exposed to high levels of the pathogens by working consecutive, full days in the indoor pool environment. No patients became ill at the facility.
The indoor pool water was kept at temperatures of 92 degrees to 94 degrees F and was treated with hydrogen peroxide rather than chlorine. The team found that pathogens thrived inside hydrogen peroxide bubbles produced at the pool surface and were dispersed as the bubbles became airborne and burst. The hydrogen peroxide also eliminated microbes competing with "Mycobacterium avium," they reported.
Health inspectors typically capture and culture microorganisms found in such environments to assess the number of pathogen species, an approach that misses many airborne bacteria that are difficult to grow in a lab environment. The new approach allowed researchers to survey more than 1,300 ribosomal RNA genes from different bacteria and fungi in the pool air and water and identify more than 600 unique sequences, including "Mycobacterium avium."
Hernandez led the sampling, and microbial analysis was done at CU-Boulder.
"This highlights the need to modernize public health methodology," said Pace, recipient of a $500,000 MacArthur "genius grant" in 2001 for his studies on the range and diversity of microbial life. "Public health progress has fallen in recent years, in part because of a shift in the nation's priorities toward bio-weapons research that diverts valuable money and resources away from very basic and important issues."
Higher winter levels of airborne pathogens in such enclosed pool areas are likely due to seasonal ventilation practices, including closed doors and windows during cold periods, said Pace. Since the study, the rate of air exchange in the pool area has been increased and additional air filters have been installed that have been shown to capture and remove up to 95 percent of "Mycobacterium avium" aerosols.
A 2004 study by Pace and his colleagues using the same molecular technique found that billions of microbes, in the form of "soap scum," can be found in a one-inch-square section of a normal home shower curtain.