A ‘Safe’ Dose?

Is there such a thing as a 'safe' exposure level to a hazard?

Is there such a thing as a 'safe' exposure level to a hazard?

In many areas of life we are exposed to hazards in the form of a substance or a process which may harm us. Mobile phones use microwaves – physically similar to those used for microwave cooking – but at a much lower power. We know that microwaves at high power are bad for us – think of microwaved meal . ANd we know that mobile phones are pretty safe – but is the exposure level from using a phone completely safe?

To the best of my knowledge, there is not a single case of a person having been harmed by using a phone (aside from as a result of the inattention they cause). But many people are still convinced that they are harmful. Indeed, for the people for whom this is a concern, the microwave radiation used by phones is intrinsically ‘bad’. And they would consider any level of exposure – no matter how low – to be dangerous. And that led me to wonder if the concept of a ‘safe dose’ really made sense?

One problem with microwaves, is that we can’t see them! And we never receive a large enough dose to sense them directly. This can lead us to either ignore the hazard (“Its all nonsense!”), or to become anxious that we are secretly being over-exposed (“They want to build a phone mast only 100 metres form my children’s school!”). So let’s first learn some lessons from two hazards with which we are more familiar: sound and light. Hazards? Surely exposure to sound and light is completely safe? Well, no. Exposure to loud sounds can cause deafness, and exposure to bright light – such as the Sun – can cause blindness. But it is not just excessively large exposures that cause problems.

Exposure to modestly loud noises does not cause immediate deafness, but prolonged exposure does reduce our hearing acuity, something we don’t usually notice for many decades. “I SAID WE DON’T NOTICE IT FOR DECADES!”. Similarly, exposure to normal sunlight causes a yellowing of the cornea which affects our vision in old age. So it is sensible to consider that exposures to sound and light should be limited so that in the course of our normal lives we are not unduly damaged.

But we also benefit from exposure to light and sound – indeed to withdraw exposure would be – quite literally – torture. As best we can tell, a whole lifetime of listening to sounds which are not-too-loud and exposing ourselves to light which is not-too-bright does us no harm and brings us wonderful benefits. So sound and light are not intrinsically ‘bad for us’ or ‘good for us’. What determines whether exposure to these ‘hazards’ is ‘safe’ is judged quantitatively, not qualitatively. For a hazard of this kind, the concept of a ‘safe dose’ makes sense, we get to enjoy the benefits and avoid the downsides.

What are the numbers for microwaves? The intensity of the microwaves inside a microwave oven is around 100,000 watts per square metre – definitely bad for you. By comparison the intensity of visible light from the full summer Sun is roughly 1,000 watts per square metre. And a mobile phone? A few millimetres from the phone surface the maximum intensity is around 0.1 watts per square metre.

Sound and light are familiar to us, and so it is easy to understand the balance required in terms of reducing our exposure to avoid future harm.  However microwaves are invisible and unfamiliar. So although the concept of determining a ‘safe dose’ quantitatively is reasonable, it is easy to understand why people feel suspicious that we are being secretly harmed. Trusting your health to a regulatory authority is not quite as reassuring as being able to trust your own judgement. But I can’t think of any alternative.

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