Diesel cars are taking a right hammering at the moment.
Dirty engines spewing out noxious fumes that are polluting our cities causing all manner of health problems is the principal charge laid out in various reports splashed across the media this summer.
Questions have even been asked about the previously unchallenged assumption that diesel engines produce less carbon dioxide (CO2) than their petrol counterparts and are, therefore, better for the environment.
But how bad is the problem and why isn't more being done to address it?
Why are people concerned about diesel cars?
In a word, pollution, which has severe consequences for everyone's health.
What is the main problem?
A number of studies have shown that diesel cars, unlike petrol cars, spew out high levels of what are known as nitrogen oxides and dioxides, together called NOx. Nitrogen dioxide (NO2) is particularly nasty - recent studies have shown it can cause or exacerbate a number of health conditions, such as inflammation of the lungs, which can trigger asthma and bronchitis, and increased risk of heart attacks and strokes.
In many European cities, NO2 levels are well above European Union legal limits - twice the limit in parts of London, Paris and Munich, according to the European Environment Agency (EEA).
Diesel vehicles are the single biggest contributor to these high levels of NO2.
Is it just NOx we should be concerned about?
No - particulate matter, which is belched out from diesel exhausts, has been shown to cause cancer. This has long been recognised, and modern diesel cars are fitted with extremely effective filters that stop almost all of this carcinogenic soot from escaping into the atmosphere.
But there are two problems. First, a lot of people remove these filters to improve fuel economy and performance. A number of specialist companies advertise removing them and it's not illegal to do so, although your car should fail its MOT without one.
Second, NO2 forms something called secondary particulate matter when it enters the atmosphere, the effects of which are not yet fully understood.
How many people does this affect?
Studies suggest that air pollution as a whole causes hundreds of thousands of premature deaths in Europe.
When you consider that road transport, and diesel in particular, contribute a meaningful chunk, the gravity of the problem becomes immediately clear.
Indeed a recent study put the number of premature deaths in the UK attributed specifically to NO2 at 23,500.
The number of people generally affected by health problems will, of course, be much greater.
And the economic cost must not be underestimated. According to the latest figures available from the OECD, premature deaths and ill health caused by air pollution cost the UK $86bn (£56bn) in 2010.
Across OECD countries, the body says road transport accounts for half the total economic cost. Of course this cannot all be laid at the door of diesel engines.
What about carbon dioxide emissions?
We've always been told that diesel cars are better for the environment because they emit less CO2. This is simply because diesel engines are more efficient than petrol engines, so use less fuel to travel the same distance. Less fuel should mean lower emissions.
But industry data shows that average CO2 emissions from diesel cars are only fractionally lower than those from petrol cars.
This is largely due to the fact that diesel cars tend to be bigger and heavier than petrol cars, so any advantages in efficiency are wiped out. Equally, diesel fuel has more carbon than petrol for the same volume, according to the International Council on Clean Transportation (ICCT) - burning one litre of diesel produces 12% more CO2 than burning one litre of petrol, it says.
Petrol engines have also become far more efficient in recent years.
For its part, the car industry itself says diesel takes longer to burn so you use less, and argues that of course emissions would be greater from larger cars. It maintains that when comparing like-for-like models, diesels do emit noticeably less CO2 than their petrol counterparts.
Why has all this been allowed to happen?
Good question, especially when you consider that diesel cars emit far more pollutants than they should. Just how much more is quite shocking, according to some studies.
Tests conducted by the International Council on Clean Transportation (ICCT) show that modern diesel cars emit on average seven times the EU limit for NOx.
Another study published by green transport think-tank Transport & Environment and supported by data from Emissions Analytics, suggests that about nine in every ten new diesel cars exceed the limit. It showed that of the 24 cars tested, only three cars - an Audi A5, a VW Golf and a BMW 3-series - complied with EU regulations. At the other end of the scale, an Audi A8 emitted 22 times the limit.
The reason is very simple. Limits are based on tests conducted in laboratories where conditions do not reflect driving out on the open road.
Why aren't carmakers doing more?
The car industry says it has done a great deal already, reducing both particulate matter and NOx emissions significantly over the past few years. And the UK leads the way in Europe, the Society of Motor Manufacturers and Traders (SMMT) says. It also acknowledges that the current way of testing "is outdated and the discrepancy does the industry no favours", while agreeing that real-world tests are needed.
But it also seems carmakers could do a lot more. The ICCT says the technologies for real-world clean diesels already exist, but are not being used consistently by manufacturers. Transport & Environments says it's simply a question of cost - carmakers save about £220 a car by not using additional clean technologies.
And the US experience suggests it may have a point. There, a concerted effort by carmakers and government agencies to clean up diesel vehicles has resulted in massive reductions in NOx, particulate matter and sulphur.
What is the government doing about it?
European regulators are in discussions with carmakers about the introduction of real-world testing. They want to bring these tests in by 2017, but they need the agreement of all member states. Carmakers would prefer more time.
Rather bizarrely, the new limits are likely to be less stringent than the current limits, to reflect real-world testing.
But individual countries are beginning to act. The UK is one of six European countries potentially facing hefty fines if it doesn't get NO2 levels down by 2020.
To this end, the government launched a consultation document last weekend, suggesting that diesel drivers in London, Birmingham, Leeds, Nottingham, Derby and Southampton could be limited driving into the city centre.
London Mayor Boris Johnson has already announced that diesel cars will be charged an extra £12.50 on top of the congestion charge from 2020 if they fail to meet emissions standards.
Charging diesel drivers higher taxes has also been mooted.
What can I do to help?
Not a great deal, unfortunately, but there are some things you can do that will help to reduce emissions, many of which apply to all cars, diesel and petrol:
- Don't accelerate unnecessarily
- Get your car serviced regularly
- Turn your engine off if you are stationary for more than one minute
- Stick to the speed limits, especially on the motorway
- Check, or get your garage to check, your car's levels of urea (effectively ammonia used to trap NOx)
- Be very careful buying any retrofit solutions - none are fit for purpose according to Transport & Environment