Significant relationships between household endotoxin and diagnosed asthma

Exposure to household endotoxin levels poses a significant risk for asthma, according to the first nationwide sampling of house dust.

The study appears in the first issue for December 2005 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.

Endotoxins are toxic substances associated with the outer membrane of certain gram-negative bacteria. These molecules are bound to the bacterial cell wall and are released when the bacterium ruptures or disintegrates. According to the authors, inhalation exposure to endotoxins is common in homes. Indoor sources include: dust, pets, humidifiers, pests, and outdoor air.

Past studies have shown that exposure can cause lung inflammation.

Peter S. Thorne, Ph.D., of the Environmental Health Sciences Research Center at the University of Iowa, Iowa City, and five associates, evaluated 2,456 residents in 831 homes selected to represent the demographic characteristics of the U.S. population. The investigators took 2,552 house dust samples from five locations within the homes, including bedroom floors, bedding, family room floors, sofa surfaces, and kitchen floors.

"This study clearly demonstrates significant relationships between household endotoxin and diagnosed asthma, recent asthma symptoms, current use of asthma medications, and wheezing," said Dr. Thorne. "No effect was observed of allergy status on the relationship between endotoxin and asthma outcomes. This suggests that current endotoxin exposure may have little impact on allergy status and that airway inflammation is the most significant effect of endotoxin exposure in a cross-section of the population."

The authors found the strongest relationship between asthma, asthma medications, and wheezing came from endotoxin levels in bedroom floor and bedding dust. However, the effects were observed only in adults and not in children.

Moreover, the investigators also noted that the endotoxin concentrations were highest in kitchen and living room floor dust, and lowest for bedding (including mattress and pillow).

"The mean concentration of endotoxin in the kitchen floor dust was 2.3-fold higher than bedroom floor dust and 4.3-fold higher than bedding dust," said Dr. Thorne.

Two field workers visited each household and took answers from residents to detailed questionnaires, conducted a home inspection, and vacuumed dust into an in-line filter.

The study yielded an 11.3 percent prevalence rate for diagnosed asthma among the homes surveyed. This figure compared favorably with the 11.1 percent prevalence rate of diagnosed asthma associated with the 2002 National Health Interview Survey.

The authors also noted that their national survey demonstrated that U.S. household endotoxin exposure levels were higher than those in Europe.