In 2012, a radio station in Sao Paulo, Brazil, started transmitting, along with its regular music broadcast, an audio signal at a frequency not audible to the human ear. The station told its listeners to stay close to the radio during the broadcast because the hidden, underlying tone repelled mosquitoes.
The radio station broadcast the high-frequency sound for three weeks, from 6-8pm—peak time for mosquitoes in the region. It claimed that while the tone was “all but inaudible to humans”, to the mosquitoes, it sounded like the flutter and imminent presence of a predator dragonfly.
According to reports, millions of people turned on their radio sets in the evening, as they sat in the open to enjoy the breeze. No one really knows how many avoided being bitten by mosquitoes, but the campaign, sponsored by the Go Outside adventure travel magazine to encourage listeners to step outside, won a prestigious advertising award in 2012.
I read the story recently on the BBC Magazine website after a member of my extended family living in the US, who is incidentally a frequent visitor to India, showed me an iPhone app called “Anti Mosquito—Sonic Repeller”. This app is free to download from the App Store on your iPhone, and there is also a premium version which costs Rs.60.
“You don’t use this app in India?” she asked me. I was zapped. I had never heard of apps that claim to repel mosquitoes. I’ve seen ultrasonic devices that claim to repel mosquitoes, but not an app.
She touched the phone screen and said, “Can you hear it?” The app claims to emit a tone at three frequencies: 14 kHz, 16 kHz and 20 kHz. The human ear has a hearing range of 20-20,000 Hz (or 20 kHz), but as we age, we find it harder to hear certain frequencies. The radio station in Brazil had transmitted the sound at 15 kHz. I tried a few online hearing tests, but the result varied from one site to another.
When I told my relative that I couldn’t hear any sound, she touched the 16 kHz button. I still couldn’t hear a thing. Later, I checked with my ENT specialist and he said that a 14 kHz frequency would sound like a shrill tone, and that as one gets older, one loses the ability to detect higher-frequency sounds.
Now whether I could hear the tone or not is not the point. The app claims to repel mosquitoes, and what I wanted to know was, can a cellphone app really do that?
I have trawled the Internet and have found no evidence that these apps work. Besides, there is no scientific evidence to prove that mosquitoes are repelled by high-frequency sounds.
The app developer’s website (Picobrothers.com) says it emits “a very unique high-frequency sound (ultrasound) that the insects dislike. The pitch of the sound is so high that most humans will not notice anything”.
The iTunes website for the app adds: “The app does not guarantee 100% protection as there are over 3,500 known mosquito species in the world and they all react slightly differently to the repellent. Use the pitch selector to find the optimal repelling frequency for the mosquitoes in your region.”
There are dozens of free and paid apps for both Apple and Android devices that claim to repel mosquitoes. YouTube has a “mosquito repellent” sound that goes on for 11 long hours. Plug-on ultrasonic devices called “Electronic mosquito repellents” or EMRs which, like the apps, claim to emit high-pitched sound, have been around for years. I saw one online for as little as Rs.139.
In an interview to the award-winning radio podcast, The Naked Scientists, James Logan, medical entomologist at the London School of Hygiene and Tropical Medicine, says: “There are many devices in the market that claim to emit high-frequency sound and effectively repel mosquitoes. The sound is meant to mimic a natural predator, such as a dragonfly, that female mosquitoes may want to avoid. The other theory is that the sound mimics a male mosquito, and if a female has mated already, she would want to avoid the male. There is no scientific evidence, though, that high-frequency devices repel mosquitoes.”
According to the American Mosquito Control Association, “at least 10 studies in the past 15 years have unanimously denounced ultrasonic devices as having no repellency value whatsoever”. So if you believe that an app can protect you from mosquitoes, remember there is no evidence of this yet.
Shekhar Bhatia is a science buff and a geek at heart.