Endotoxin Levels High in Indoor Equine Environments 

Researchers measured the concentration of endotoxin in indoor buildings associated with horses.
Horse stable, where there might be high endotoxin concentrations
Researchers found the highest levels of endotoxin concentrations in stables with many horses. | Adobe Stock

Endotoxin is a substance known to incite respiratory allergies. In horse environments, such as stables, arenas, and horse trailers, endotoxin levels are high. Researchers in Japan—a country known for its high incidence of asthma in humans—measured the concentration of endotoxin in indoor buildings associated with horses, including an old Japanese house, an accommodation facility connected to a horse stable, and Thoroughbred racing stables. High levels of endotoxin exposure in these kinds of environments can have significant effects on both human and equine respiratory tracts.  

Risk Environments

Old Japanese houses, once used for farming with horses, are L-shaped, with the horse area and human living space connected by an entrance hall. These houses are now tourist attractions, and horses are tethered to the buildings to approximate a traditional Japanese living environment.  

The second type of risk environment for endotoxin is accommodation connected to stabled horses. Horses and humans co-exist, with the human spaces connecting directly to the stables through an entrance hall. Horses are turned out to graze and do not necessarily return to the stables at night.  

The third environment is a racehorse stable that provides equine housing for rest, training, and riding. The upper floors of the stable are living spaces for horse trainers and their families. Researchers measured airborne concentrations immediately and within 30 minutes following cleaning and bedding 12 stalls occupied by horses. They also measured concentrations at a second Thoroughbred training facility surrounded by mountain forest with 20 stalls occupied by horses. The facility has a stable for the horses, a rest and reception area for staff, and a second-floor attic. 

Research Findings

The researchers found that the presence of horses in a building is associated with higher air concentrations of endotoxin than buildings without horses. The highest concentrations were found in stables with many horses compared to the old house environments or accommodations with fewer horses. This is due to the higher density of horses and endotoxin in straw bedding. 

Dust concentrations varied in different rooms within the same building, possibly due to effects of floor materials, cleaning frequency, and foot traffic from areas of high endotoxin concentrations. After stalls were cleaned and bedded, endotoxin air concentrations were very high. Workers are potentially exposed to high endotoxin concentrations while cleaning and bedding stalls. 

These results might be useful for individuals trying to control endotoxin concentrations in indoor equine facilities. 


Ogasawara T, Kim H, Lim E. Study on endotoxin in indoor environment: research at equine-related facilities in Japan. Asian Journal of Atmospheric Environment 2024, vol. 18:3; DOI: 10.1007/s44273-023-00025-6

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