From a review at Amazon
An amazon review of another model said several members of her family got headaches with it.
We put 10 of these in our cabin and I got a headache in 24 hours .
From (118) Proof That Ultrasonic Pest Repellers Are A Scam: Class Action Lawsuit For Fraud. Mousetrap Monday. - YouTube, which tested another brand.
They also say the Bell + Howell has the most misleading advertising and has had a class action law suit filed against it.
5 Best Ultrasonic Pest Repellers - Mar. 2021 - BestReviews
It’s important to note that this type of device is not effective against all pests. Certain insects, like ants, are more resilient than others when it comes to ultrasonic frequencies. Some studies have also shown that ultrasonic repellers may even attract critters (such as mosquitos) into your dwelling. Correct positioning and use of these units is the best way to maximize effectiveness, but don’t think of it as an instant solution to pest problems. It takes time for repellers of this type to produce a measurable effect.
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