The London Low Emission Zone (LEZ) was introduced in 2008 in an attempt to curb the city's traffic-related air pollution, but it seems that its early years weren’t powerful enough to really improve air quality. A recent paper in PLOS One found that schoolchildren living in the LEZ had the same exposure to pollution during the first three years of the policy, and have continued to suffer from health conditions that are associated with pollution.

The LEZ targets only larger diesel vehicles, which are required to pay a fee to enter the zone. In 2008, the Euro 3 standard of emissions was required, but in 2012 it was stepped up to the stricter Euro 4 emissions standard. Assessing the impact of each progressively tighter level of control, on both air pollution and real-world health effects, can contribute evidence-based and effective traffic pollution policies.

Health snapshots

A group of researchers led by Helen Wood at Kings College London surveyed the health of children in Hackney and Tower Hamlets. They focused on children around 8 to 9 years old because they wanted to examine a population whose lungs were still developing and who were likely to be badly affected by pollution.

They contacted children via their schools, and found slightly more than 1,000 families that were willing to participate. Children underwent a health assessment, and families filled in a survey about the children’s experiences with asthma, allergy, and eczema. The researchers also assessed how much air pollution the children were exposed to at their home addresses.

The tricky thing with surveys is that people can recall things incorrectly or exaggerate the answers, so survey data requires careful assessment. In this case, the surveys actually reported slightly less respiratory illness than a previous country-wide study, so exaggeration doesn’t seem to have been a problem.

It’s important to note that the researchers didn’t go back to the same group of children again and again over three years, which would have allowed them to see whether the same children were getting healthier or sicker. Rather, they visited each school once, going to different schools over a period of three years. It’s a choice of method that is faster and cheaper, but likely to result in slightly hazier information about whether or not things really improved.

Consistently bad air

Over the first three years of the LEZ, levels of illness didn’t decline among the children—they stayed just as sick throughout. When they looked at how much air pollution the children were exposed to in their home environments, they found that it had changed very little. So, the air wasn’t getting any better, and neither were the children.

The obvious question here is what went wrong. There’s evidence for a link between air pollution and health, and the LEZ targeted the biggest culprits of air pollution, so it’s important to figure out why the logical intervention didn't work. The authors of the paper suggest that it’s because the most stringent restrictions hadn’t yet been put in place—they say we might start seeing benefits that emerged during the last three years of more robust controls.

There are other problems that the policy doesn't currently address, including an ever-increasing number of diesel vehicles in general, and the difference between the real-world emissions of vehicles and their idealised emissions. A vehicle that passes muster on paper might actually emit more pollution than we expect in everyday driving conditions.

“It is of significant relevance to policy makers to note that the LEZ, which was designed to reduce traffic-related emissions in London, has not actually done so up to this point,” the authors write. Their study on schoolchildren is ongoing, and may still pick up on an improvement from the later, stricter years of the LEZ.

PLOS One, 2015. DOI: 10.1371/journal.pone.0109121 (About DOIs).