Pests cause damage, spread disease, and at best are just a nuisance. They come in all shapes and sizes, and dealing with them can be time consuming and oftentimes costly. For this reason, ultrasonic repellers have become very popular with home owners. They are easy to set up and use, require little to no maintenance, and are very economical compared to many other pest control solutions.
However, apart from repelling or deterring pests, there have been increased concerns about the dangers of ultrasonic pest repellers and how safe they are for humans. Unfortunately, few studies have been carried out on the effects of ultrasonic frequencies from these devices on humans, and there are no firm regulations or guidelines in place. Hence, consumers must for now, rely on and trust, that manufacturers will use their best judgment.
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How Do Ultrasonic Pest Repellers Work
Ultrasonic pest repellers are electronic devices that emit high frequency sounds, above the audible range of humans. They are designed to repel, injure or kill household pests, such as rodents and insects. The ultrasonic (or ultrasound) frequency range starts at 20kHz, although if the sound level is high enough, frequencies up to 24kHz may be audible by some people.
There are various ultrasonic pest repellers available on the market, designed to deal with pests inside the house as well as outside. For the garden, ultrasonic pest repellers are battery powered, with many also being solar powered. They work by emitting an ultrasonic frequency that is annoying to burrowing pests like moles, voles, and gophers, forcing them to relocate elsewhere.
Inside the home, the devices plug into any electrical outlet, and the ultrasonic frequency they emit, causes audiogenic seizures, intended to deter mice, rats, cockroaches, mosquitoes, and other unwanted pests.
Audiogenic seizures (AGS) are generalized convulsive seizures that are induced, by a loud sound. AGS in rodents involves an interaction between two networks, the brainstem auditory and locomotor networks. It is characterized by wild running, clonus (involuntary and repetitive bouncing), tonic extension of the limbs, convulsions, and in rare cases, death from cerebral hemorrhage.
An early study (1984) titled “The effect of changes in sound pressure level and frequency on the seizure response of audiogenic seizure susceptible rats”, carried out at the University of Kuopio, Kuopio, Finland showed the relation between frequency and the sound level required to induce AGS – 110dB at 5 kHz, 97.5 dB at 10 kHz, and 83.8 dB at 20 kHz. The higher the frequency, the lower the sound level required. The study further concluded that, “repeated exposure to seizure-inducing sound at weekly intervals led to more severe seizures”.
Learn More: The Different Types of Pest Control Technology
What are the Concerns for Humans
A large number of experiments and studies carried out by research laboratories and the scientific community in general, use laboratory rats and mice. There are many reasons why scientists use these rodents, among them being that they are small and easy to handle, they have short lifespans, making it is easy to observe the effects on several generations over a relatively short period of time, and they can be inbred so that genetically they are almost identical, helping keep results of medical trials more uniform.
Another reason they’re used as models in medical testing, is that their biological, genetic and behavioral traits, are a close match to those of humans. That being the case, it is quite reasonable to assume that any dangers to mice and rats caused by ultrasonic pest repellers, could potentially also be harmful to humans, although perhaps not to the same degree.
While studies have been carried out on the effects of occupational exposure to ultrasound devices and equipment used in the workplace (e.g. industrial ultrasonic cleaning equipment, ultrasonic drills, medical diagnostic devices and therapeutic applications in hospitals and medical facilities), very few scientific studies are available that have examined possible adverse effects from devices used in public areas and the home, especially concerning any potential dangers of ultrasonic pest repellers, which have become quite popular and widespread.
General reports on the effects of protracted exposure to ultrasonic devices include headaches and dizziness, migraines, fatigue, tinnitus, balance disturbances, and nausea. Furthermore, despite ultrasound being inaudible to humans, ultrasonic frequencies can also contribute to hearing loss.
According to an article from the Belgian Federal Public Service (FPS) Department of Health, Food Chain Safety and Environment, “if the ultrasound is too strong, sub-harmonic vibrations are generated in the ear, which are audible and which can cause hearing damage”.
Studies Showing Adverse Effects
There are a number of factors, among them age and an individual’s sensitivity to sound, that contribute to the adverse effects from exposure to ultrasonic frequencies, either in the short term or for protracted periods of time.
One study, titled “Assessment of short-term exposure to an ultrasonic rodent repellent device” (https://asa.scitation.org/doi/pdf/10.1121/1.5063987) carried out at the University of Leuven, Belgium, on 25 young and 25 middle-aged participants, concluded that no significant adverse effects were observed when the subjects underwent short exposure to frequencies produced by the repellent device.
The device used was a KEMO ultrasonic repellent device (type M175-02-18, KEMO Electronic GmbH, Geestland, Germany). While the device has 10 different settings, three frequency settings were evaluated, with nominal frequency ranges of 12–14 kHz, 25–28 kHz, and 35–41 kHz, and levels of exposure lower than 100 dB Sound Pressure Level (SPL).
Spectral analyses showed that apart from the primary frequency generated by the device, a very low signal was also present at 4 – 5 kHz, which is well within the audible range of human hearing. In fact, even when the device was set to operate at the 25 – 28 kHz and 35 – 41 kHz frequency ranges (frequencies that are outside the audible range for humans), some participants could perceive a high frequency sound, especially when two sound sources were used.
Another study, “Investigation on high-frequency noise in public space”, measured the noise from a rodent repeller device installed in a commercial facility. The study was also accompanied by a survey in the form of a questionnaire, conducted on staff at the facility.
Measurements taken during the study, showed that the maximum SPL was 120 dB directly next to the device, dropping to 90 dB at a distance of 15m from the device. Regarding results of the survey, it was found that younger staff could more clearly discern a high pitched sound than could their older colleagues, and that it created an undesirable effect that was characterized as unpleasant, noisy, and even causing headaches or earaches.
A very detailed article is available from the National Institute of Health (NIH) which addresses the issues of what evidence has been available to date and what guidelines are currently in place, with the aim to show that the public is indeed being exposed to varying doses of ultrasound, and that there is inadequate data available to make any form of solid assessment on the effects on safety from such exposure.
Final Thoughts
With many of the studies examined, surveys were also included as part of the study, as this was the only way to determine if any discernible adverse effects were experienced directly by the participants.
This of course relies on the hearing sensitivity of each individual. What is not known, is if adverse effects can be caused by frequencies just beyond the human audible range, effects that may not be immediately evident or attributed to ultrasound frequencies.
It was also evident that the younger aged participants were more likely to pick up high frequency sounds, even if only barely audible, that caused some degree of discomfort, either mild or more intense, such as headaches.
While there are studies on the hazards presented by equipment emitting ultrasound frequencies used in industry, professionals using this equipment are aware of the risks and can take appropriate measures of protection, such as avoiding long or continuous exposure, as well as wearing some kind of suitable protection.
This is not the case with small office or household gadgets and devices, like pest repellers, which operate around the clock, and may use or produce an entirely different set of frequencies. It would also not be practical to expect office staff or family members to walk around wearing hearing protectors such as earmuffs and earplugs all day (and night) long.
A number of factors appear to be common among most studies. The level of noise generated (or Sound Pressure Level), has a direct correlation to any adverse effects experienced by participants, especially when the level is above 110 dB.
It must also be stated that the sensitivity of the ear to high frequencies (i.e. frequencies at the high end of the human audible range), differs from individual to individual, and in general, the younger the person, the greater the sensitivity, and therefore the more likely they are of picking up high frequencies, and even frequencies above 20 kHz.
Age is therefore also a consideration, where greater high frequency acuity in young people makes it difficult to set a single, safe cut-off frequency to cover the whole population.
The Sound Pressure Level drops off with distance from the source. However, in many applications, several devices may be deployed to cover a larger area, and there may be hot-spots where the cumulative sound level can be quite high.
While exposure for short periods does not appear to cause adverse effects, protracted and repeated exposure does. A household ultrasonic pest repellent is on all the time and would therefore fall into the latter case.
Finally, there is enough anecdotal evidence to indicate that there are legitimate concerns over the dangers of using ultrasonic pest repellers, and that further scientific studies need to be carried out that can then be used to set specifications and guidelines for manufacturers of these devices to follow.
