Australians love their smartphones: 88 per cent of us own at least one, and a whole generation has never known life without them.
But with pervasive smartphone use and associated radio waves they need to work, are there any health effects?
One of the first scientists to pose this question was Australian physiologist Ross Adey in the 1970s.
While mobile phones were yet to gain the ubiquity they have today, Professor Adey was concerned by the frequencies emitted by household appliances and electric power lines.
In experiments conducted in the US, he found rabbit brain tissue was sensitive to very weak electromagnetic radiation.
Professor Adey was, unsurprisingly, a controversial figure until his death in 2004, and few of his experiments have been replicated.
But in the decades since that initial research, loads of time and money have been spent looking into how electromagnetic radiation might affect our health, from sleep patterns to cancer.
Let’s start with the basics.
We are surrounded by all sorts of different types of electromagnetic radiation every day: your eyes pick up visible light, your bag is scanned by X-rays at airport security, microwaves heat your lunch and too much ultraviolet light gives you sunburn.
At its essence, electromagnetic radiation is energy comprising an electric field and magnetic field, which travel together, but perpendicularly, in waves.
Sometimes the length of these waves (or wavelength) is very short â€” a few nanometres for X-rays â€” while others are much longer â€” a few centimetres up to kilometres.
It’s these long wavelengths, called radio waves, that are the electromagnetic radiation of choice for mobile phones and base stations.
Unlike shorter wavelengths, such as visible light, radio waves can pass through walls. The longer the wavelength, the better it can penetrate solid stuff.
Another term you might see is frequency, which is the number of times a wave makes a full oscillation each second.
Frequency and wavelength are closely related. Wavelength is the speed of light divided by the frequency, so long wavelengths also have low frequency.
The radio frequency end of the electromagnetic spectrum is home to what’s known as “non-ionising radiation”, said Rodney Croft, from the University of Wollongong and director of the Australian Centre of Electromagnetic Bioeffects Research.
It’s the high-frequency, short wavelength radiation, such as X-rays, that can tinker with your DNA and are linked to cancer.
These waves are small enough and carry enough energy to knock electrons off atoms, ionising them.
Radio frequency used in mobile communications simply doesn’t have the energy to do that. But that’s not to say it doesn’t exert any effects on the matter it travels through.
“It’s an oscillating wave, which swings between positive and negative,” Professor Croft said.
“If you have a bunch of molecules rotating, that causes friction, and energy is given off as heat. It’s how a microwave oven works.”
In Australia, mobile phone and base station exposure limits are set by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and regulated by the Australian Communications and Media Authority.
The ARPANSA standard looks at how much energy a user absorbs from a mobile phone over time.
The maximum limit is currently 2 watts per kilogram of tissue. Phone manufacturers usually specify their maximum absorption rate in their manual.
You can find how much radio frequency is emitted by base stations at the Radio Frequency National Site Archive.
According to ARPANSA, it’s “typically hundreds of times below the [ARPANSA] limits”.
So are we microwaving our head whenever we answer the phone? A tiny bit, but not enough to be worried about, Professor Croft said.
He and his team found mobile phone radiation exposure increases the temperature of the outer grey, wrinkled layer of the brain called the cortex, but it’s only “maybe about 0.1 degree, which is very small compared to the temperature variation the body normally has to contend with”, he said.
“We do find that we get a slight change to thermoregulation, so the body, even with that small change, is sending a bit more blood out to the periphery to cool it, so your body doesn’t end up warming up.”
What about activity in the brain?
In work published in 2008, he and his colleagues held a Nokia phone to the head of healthy participants and monitored their brain waves.
What they found were changes to a type of brain activity called alpha waves, which are associated with relaxation, but the effects were incredibly subtle: mobile phone exposure enhanced alpha wave activity by around 5 per cent.
“Normally, if you close your eyes, you might double your alpha activity,” Professor Croft said.
“So the [mobile phone] effect is very, very small relative to quite mundane functions, and that’s why we’ve been doing a bit of work to find out if there are any functional consequences.
It’s also important to remember that this work was done way back in the days of second-generation, or 2G, cellular technology, said Professor Andrew Wood, a biophysicist at Swinburne University of Technology.
“2G used quite long wavelengths. Now the wavelengths are shorter, and the radiation doesn’t penetrate [the head] as much.”
Researchers have looked to the plant world too, to see if radio frequency stunts growth or makes the plant flourish.
For instance, Malka Halgamuge at the University of Melbourne bathed germinating soybeans in mobile phone or base station radiation.
When they measured how quickly the seedlings grew, she and her colleagues found the plants exposed to base station radiation grew longer roots and a shorter shoot compared to their non-radiated counterparts.
But look outside, and it’s obvious that their results in the lab don’t reflect the real world.
“If you observe the area around a base station in Australia, you don’t see shorter trees around the base station and taller trees further away,” Dr Halgamuge said.
So what’s behind the discrepancy?
Dr Halgamuge suspects it’s because plants â€” like us â€” have built-in systems that mend any minor damage that low-level radiation might inflict â€” even if it’s long-term exposure.
Mobile phones are classed as “possibly carcinogenic” by the International Agency for Research on Cancer, putting them in the same category as aloe vera, bracken fern and Asian pickled vegetables.
And while radio frequency is non-ionising radiation â€” remember, it can’t knock electrons off atoms, fiddle with genetic material and trigger tumour growth like ionising radiation can â€” studies still investigate possible links.
Research published in February this year by the US National Toxicology Program found tumours grew in the nerves around the heart of male rats if they were bathed in extremely high levels of mobile radiation.
But, Professor Croft said, “there were so many difficulties with that study.
“As a result, their animals were getting very hot and their core temperature was very high over their entire lifespan.”
The National Toxicology Program scientists acknowledged that their “findings should not be directly extrapolated to human cell phone usage“.
An aspect of the study that wasn’t widely reported, Professor Wood added, was the apparent protective effect of mobile radiation on the rats.
“The control rats didn’t live as long as the rats that were exposed to lower levels of radio frequency.”
Some research linking cancer to mobile phone radiation is in the form of “case-controlled studies”, said Simon Chapman, a former public health researcher at the University of Sydney.
These quiz people with and without brain cancer about their mobile phone habits, and compare the groups.
But there’s a flaw in the protocol: the human memory isn’t as good as we’d like to think it is.
“It’s hard enough to remember when you got your first mobile phone, let alone how much you used it back then,” Professor Chapman said.
So, in 2016, Professor Chapman looked for any increase in brain cancer incidence in the decades after mobile phones were introduced to Australia in 1987.
Today, there are “far more mobile phone accounts in Australia than there are people, that’s even allowing for infants and extremely old people”, Professor Chapman, who since retired, said.
“It’s saturation level usage, and it’s been like that for a long time.”
But they found no increase in the number of brain cancers reported from 1982 to 2013, even in the years after mobile phones grew in popularity.
“We used a conservative expected incidence, should mobile phone radiation cause cancer â€” a 50 per cent increase,” Professor Chapman said.
“The only age group which showed there was an increased incidence of brain cancer was in the very, very oldest age group. But that increase started before mobile phones were available in Australia.”
The reason behind this increase, he added, is probably because more sophisticated brain diagnostic techniques picked up tumours which might have previously slipped by unnoticed.
It’s coming in the next couple of years and promises to bestow lightning-fast download speeds. So does this mean it’s dangerous?
It’s a common question, Professor Croft said: “People might say ‘OK, we’ve seen from research that 4G is alright, but what about 5G?'”
The only real difference between the two is 5G uses a higher frequency. And with a higher frequency comes a shorter wavelength, and short wavelengths can’t penetrate solid stuff as well as longer wavelengths.
“Energy will be concentrated in the skin itself, rather than penetrating a bit deeper. It won’t go into your brain.”
To that end, Professor Wood’s team is building models of skin to predict how energy from mobile radiation will be absorbed.
Despite research showing no link between safe levels of radio frequency and cancer, telecommunications companies and other organisations do offer suggestions if you want to reduce exposure.
The obvious action, Dr Halgamuge said, is to limit mobile phone use: “You have no control over base stations, because that radiation is around you all the time, but you do have control over your mobile phone.”
The ARPANSA also recommends using hands-free or texting instead of calling, “but none of those things are actually based on any health effects”, Professor Croft said.
Then there are companies that make devices claim to block “harmful mobile radiation”, he added: “But of course if it’s cancelling [radio frequency] out, then your phone can’t work.”
So: does radio frequency have any effect on human tissue, apart from heating it a fraction of a degree?
That question is still open, Professor Wood said.
“Even though some of the evidence [that radio frequency causes damage] on the face of it looks quite compelling, there’s still a question of consistency.
Other researchers suggest that health risks associated with mobile phones may be indirect, “such as the sharply increased incidence rate of traffic accidents caused by telephony during driving, and possibly also by stress reactions which annoyed bystanders may experience when cellular phones are used in public places”.
What is certain, Professor Wood said, is that radio frequency is a life-saver.
“While I don’t think [radio frequency] emerges with a completely clean bill of health, it emerges with a pretty good bill of health.
“The fact is that it saves lives. Think of search and rescue and emergency situations. The benefits far outweigh the risks.”