I saw this Tweet on my feed this morning and immediately knew something was wrong.

Let me be clear, I totally agree with the aims of any such campaign but, certainly this suggestion, I disagree with. I’ll explain…

First of all, let’s look at what they’re attempting here.

By turning off your car engine whenever you’re not moving – and it’s safe to do so – you’ll help to make the air cleaner for you, other drivers and pedestrians.

Now, for the sake of clarity I’m now going to discuss petrol cars. They make up the majority and I think we can all agree that diesel cars are generally pretty polluting anyway. Besides, I’ve never owned one so my knowledge on how they run is lacking.

Right, now that’s out of the way, let’s talk about the catalytic converter that’s on every petrol car. It’s a wonderful device that converts pollutants to water. However, there is an issue with it – it needs to be hot to work. A cold car is a polluting car. So, switching off your engine, allowing it to cool (or maybe not even allowing it to get up to temperature in the first place), is not a good idea.

So, straight off the bat, this may not be the best advice. If you car is hot and your ‘cat’ is doing its job then the amount of pollutant produced will be minimal.

Then, there’s the power usage. Starting a car takes a huge drain on a battery – on a cold, winters day you may have your lights and heater on, all things which need a lot of power. If you keep restarting your engine, this will have a big impact on your battery, meaning that your car will be dragging the power directly via the alternator, impacting fuel usage negatively. Basically, keep restarting your engine and you’ll use more fuel, increasing your use of fossil fuels and, more importantly in this case, increasing your pollution.

But, there’s something else. Most modern cars have start/stop technology – if you have this you won’t need to follow this advice. But, of those cars that have this, they also have other modifications to cope with the constant starting and stopping – a stronger starter motor, for instance.

A standard starter motor is designed to start/stop 50,000 times in its life. It’s estimated that those with start/stop technology may do this up to 500,000 – hence the change to the starter motor. And this is just one of many engine changes that have to be made. Yet, what’s being advised here is to reproduce the same effect but without any such modifications being made to your car, potentially burning through your starter motor, crankshaft and other components. Or as Auto Car puts it, when talking about start/stop technology…

The higher number of stop-start cycles lead to increased engine wear unless steps are taken to prevent it.

Yet, this is at odds with the suggestions on the National Clean Air Day Campaign website. Where point 2 is…

#2. Switch your engine off when stationary

Point 7 is…

#7. Regularly service your car

Remember to service your car regularly to make sure it runs as efficiently and cleanly as possible.

To me these two points are at total odds with each other.

So, if you have a petrol car and don’t have start/stop, I’d actually say following this advice is dangerous and possibly more polluting in the long term. Ignore it. Buy a better car, go electric, wind up your window or do one of many other things that will help – but, please, don’t do this.