Units are important.

Units are how we measure things. They are how we know whether a cop is going to write us a speeding ticket, whether we need to try harder at that diet we’ve supposedly been on since New Year, or whether the bloke who owns the petrol station has pulled a fast one on us.

Official and legal units are carefully defined by measurement scientists (metrologists), but that hasn’t stopped weird and wacky units from being invented, often because they are more convenient to use than the official ones.

I’ve written before about some of my favourite units. (I’m a metrologist. I’m allowed to have favourite units.) But there were a few I missed out that deserve some attention.



We humans are notoriously bad at understanding how dangerous something is. How deadly is smoking compared with air travel? Should you be worried about being eaten by a shark when you go to the beach, or are you more likely to die on your way to the beach? In 1989, Stanford professor Ronald Howard invented the micromort as a way of comparing the relative risks of certain actions.

If a particular action has a one-in-a-million chance of killing you, then that action or activity carries one micromort of danger. Because it’s based on statistics, something’s micromort count will change over time, but it still provides a useful way of comparing the relative danger of different actions and choices.

As a surf life saver with more than a decade of experience, being attacked by a shark is not something I give any serious consideration. The risk of shark attack while swimming in the ocean in Australia contributes only around 0.125 micromorts of danger over a year. You are just as likely to be kicked to death by a kangaroo, and far more likely to die in a car accident on your way to the beach.

We tend to fear shark attacks far more because they are rare and dramatic while we ignore more mundane but significant dangers. We have a very distorted perception of what is dangerous and what isn’t. Around Australia, local governments waste millions of dollars on anti-shark schemes, money that could save many more lives if allocated more sensibly. The same considerations also need to be applied to issues ranging from pollution to terrorism. Micromorts go some way to helping us overcome our irrational perception of risk.

You are more likely to be killed by the chair you are sitting on.


Banana Equivalent Dose

Radiation is scary. But everything is radioactive. You, me, the cat, rocks, the food we eat, everything gives off low levels of radiation. This is our natural background radiation and is harmless to us. But the word “radiation” is so feared that as soon as it is mentioned it becomes very difficult to communicate the real risks, or lack thereof, of minor doses of radiation, and the official unit, the sievert, doesn’t really help. The Banana Equivalent Dose (BED) helps to communicate the science.

A surprisingly useful unit of radiation dose.

Bananas are rich in potassium and, because natural potassium on Earth includes small amounts of radioactive potassium-40, this makes bananas slightly more radioactive per kilogram than your average food item. (My favourite thing about this is that potassium-40 occasionally produces anti-electrons. The average banana produces 15-20 pieces of antimatter per day.) One average banana gives you a dose of about 0.1 microsieverts of radiation, which is about 1% of your average daily background radiation dose, so your daily average dose is 100 bananas.

A chest x-ray will give you about 200 bananas, a cross-country flight will give you 400 bananas, and spending a day near Chernobyl will give you 60,000 bananas, which is still less than a CT scan, which will give you 70,000 bananas. Radiation workers (nuclear power plant employees, x-ray technicians, astronauts, etc) in the U.S. are allowed a maximum dose of 500,000 bananas over a year, and a lethal dose of radiation is about 35,000,000 bananas. Hopefully, this measure puts some things, like using an old CRT computer monitor for a year (10 bananas), in perspective. (It’s also interesting to note that, in the U.S., living within 50 miles of a nuclear power station gives you about 0.9 bananas over a year, but living within 50 miles of a coal-fired power plant gives you 30 bananas over a year. This is because radiation at a nuclear plant is contained, while the radioactive elements present in the coal are just belched out into the air.)

Randall Munroe, the creator of xkcd, created a radiation dose chart that has proved to be so useful that nuclear regulatory authorities from around the world link to it for the purposes of public education and information.

Randall Munroe’s Radiation Dose Chart

Unfortunately, this isn’t always effective. The knowledge that bananas are radioactive has lead a lot of misinformed health bloggers to claim that eating too many bananas can be dangerous. (In case you don’t want to read the linked article, you need to eat around 42 bananas in a day to get potassium poisoning, and 35 million to get a lethal dose of radiation.)


Ton of refrigeration

A ton of refrigeration is, like horsepower, one of those intuitive units that aided in the transition from an old to new technology and really should have died out long ago.

Prior to mechanical refrigeration, food storage and air conditioning used actual blocks of ice to provide the cooling power. A ton of refrigeration is the amount of cooling a short ton (907 kg in real money) of ice could achieve as it melted over 24 hours. As ice making machines, mechanical refrigeration, and air conditioning became more widespread in the late 19th century, measuring a system’s performance by comparing it to the familiar ice made life easier.

But that was a century ago and, as you would expect, this measurement has fallen out of use around the world, except for North America, where for some inexplicable reason it is still used to quantify the heat-removal performance of large commercial refrigeration systems. Tons of refrigeration was useful, but it has overstayed its welcome and it is time it was retired.

And while we’re at it. We should get rid of BTUs as well. What is this? The Victorian era?



OK, this one isn’t actually on the list because it’s a favourite of mine, but because it is so widely used, despite it being almost useless. I said earlier that weird and unofficial units often get used because they are more convenient than the official units, but elephants are in no way more convenient than kilograms as a unit of mass. Elephants and blue whales are the go-to units of mass for journalists and popular science writers who are trying to find an engaging way to convey a conceptually difficult quantity of mass to an audience, but they have become over used and with so little care that they don’t really help. Telling me something is as heavy as 80 elephants or 10 blue whales is no help whatsoever because I have no real clue about how much these animals weigh. And when the science writers try to convey pressure by telling me that it’s equivalent to 8 elephants balancing on a pair of high heels, it just gets less clear. Stop using pachyderms and cetaceans as units of measure and just give me the numbers in normal units. Even if I can’t understand them conceptually, converting them to elephants doesn’t tell me any more than “it weighs a lot.”

Besides, it’s rude to ask an elephant her weight anyway.

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