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Prior to 1914, animal hair was used as a binder in most plaster walls. While horsehair plaster is strong, fire-resistant, and acts as a great sound barrier, it also comes along with a risk—the contraction of anthrax. If you've just purchased a historical home and discovered that you've got horsehair plaster walls, here's what you need to know.
How Great Is The Risk?
Anthrax is a disease caused bacteria. The bacteria are found in soil and can infect many types of livestock. An animal that is infected can transmit the disease to a human via anthrax spores, which can be found both in the meat and hair of the animal. There is documentation that anthrax spores have infected humans even after lying dormant for more than 70 years.
Since your plaster walls contain horsehair, the risk of contracting anthrax from them does exist. However, there have been no recorded cases of people contracting anthrax through working with horsehair plaster. Of the people who have contracted anthrax via other forms of exposure, 95 percent of them contracted cutaneous anthrax—an easily treatable form of the disease in which exposure happens through skin lesions. The lethal form of the disease, pulmonary anthrax, occurs when anthrax spores are inhaled, but your body can fight off some pulmonary anthrax spores on its own; you'd need to ingest thousands of them in order to actually become infected by the disease.
What Can You Do About It?
Now that you understand that there is a very small risk that your horsehair plaster walls could contain anthrax spores, you likely want to know what you should do and shouldn't do about it. What you shouldn't do is replace your walls.
Replacing Your Plaster Walls Doesn't Eliminate Your Risk. Getting rid of your horsehair plaster walls altogether doesn't actually lower your risk of contracting anthrax. Because the spores can become airborne during the tear-down of your existing walls, this course of action may actually boost your risk of contracting the disease. Furthermore, replacing your authentic horsehair plaster walls could lower the resale value of your historic home.
A better plan. Your best option for dealing with the minuscule risk of contracting anthrax through your horsehair plaster walls is to keep those walls in tip-top shape. In old lime plaster applications, hair was incorporated into the base layer or two of plaster, but there was a finish layer applied on top of that that contained no fibers. In well-maintained old plaster walls, the horsehair is encapsulated by this finish layer and poses no risk of releasing dangerous anthrax spores into the air.
Inspect your plaster walls regularly for any signs of damage. If you find a crack or hole in your wall, hire a professional plaster repair service, such as Painting By Jerry Wind, to come in and take care of it. Make sure that the company you hire has experience with plaster in historical homes; lime plaster acts much differently than the gypsum plaster used in most modern building projects. By taking care of any cracks or holes in your horsehair plaster walls right away, the layer of plaster that could potentially pose a risk will remain undisturbed and unable to expose you to any ancient anthrax spores that happen to be hiding deep within your walls.
If one of your walls is damaged all the way down to the base layer, your plaster repair specialist may ask that you leave the home while they patch up the damage. In the process, they'll be sure to wear protective gear and run a HEPA-certified air filter that removes any potentially dangerous plaster dust from the air.
If you're wondering whether or not the old horsehair plaster walls in your historical home pose a risk, the answer is probably not. While you could technically contract anthrax from your walls because they have horsehair in them, nobody has ever contracted the disease in this fashion to date. Instead of destroying the historical significance of your home by replacing its authentic walls with drywall, just inspect your walls regularly for damage and have them repaired promptly if you find any.