There's an error in the link for the Lancet editorial, but it seems to be difficult to get the link (after correction) to work properly, so I've posted the Lancet editorial text in full below:
"Scale of Europe's air pollution problem demands more action
Published: 03 December 2016
In December, 1952, London was gripped by a “great smog” that wreaked havoc on the city for days and resulted in several thousand deaths. The severity of the event provided a wake-up call to legislators and prompted a series of regulatory changes to address the problem of air pollution in UK cities, including the landmark Clean Air Act of 1956. 60 years on, however, the health effects of air pollution are still unacceptably high. In 2013 alone, exposure to one type of air pollution—particulate matter less than 2·5 µm in diameter (PM2.5)—was estimated to be responsible for almost 40,000 premature deaths in the UK. Overall, data presented in the Nov 23, 2016, European Environmental Agency (EEA) report on air quality in Europe indicates that air pollution is responsible for an estimated 467,000 premature deaths each year across 41 European countries.
Most premature deaths from air pollution are caused by cardiovascular, cerebrovascular, and respiratory disease, but there is growing evidence that air pollution might have much broader effects, including on preterm birth, fertility, diabetes, childhood neurological development, and adult neurological conditions. The EEA report outlines that despite notable improvements in some sectors, air pollution remains the single largest environmental health risk in Europe. Key European Union (EU) and WHO recommended exposure limits are being exceeded in several urban centres across Europe and will continue to do so unless current trends are drastically improved on.
So what must be done? Air pollution is a problem that crosses disciplines and borders, and will necessarily require multidisciplinary solutions that engage urban planning, public health, law, and cultural change. On Nov 23, 2016, the EU had an opportunity to drive such change when Members of the European Parliament voted on updated National Emission Ceilings. The newly agreed targets will likely cut premature deaths from air pollution by up to 50% by 2030—an improvement, yes, but considering how many lives will continue to be prematurely lost each year, it is clear that these aims do not go nearly as far as they should."
It's a bit cheeky for the Lancet to conflate a historical event where a significant number of actual deaths occurred, the 1952 London Smog, with contemporary air pollution that is being argued to cause 'premature deaths' (or 'early deaths' or 'shortened lives'), in which the prematureness of the deaths is unquantified.
The 1952 London Smog was apparently initially seen as being just another smog event that affected London every few years, until it began to be noticed that undertakers were running out of coffins, and florists were running out of flowers for funerals, as described in this BBC article from 2002 which marked the 50th anniversary of the event:
The BBC article also has a go, in the final section of the article, at linking the 1952 smog with contemporary air pollution in the city.
But if you're going to use an argument about something causing premature deaths, then that argument probably applies even more for mental health issues than for physical health. It seems to be generally recognised (whether it is true or not, I don't know) that people who are unhappy or stressed might suffer a premature death. Now the UK's membership of the EU is definitely known to have caused some unhappiness and stress to the more elderly section of the UK population, and this might be argued to be contributing to some premature deaths within this section of the population.
Now doing a quick check on what the Lancet's position was in regard to the recent 2016 EU Referendum, it was, as might be expected, to argue that the UK should remain in the EU:
So the Lancet seems to be concerned about premature deaths in the case of air pollution, but isn't bothered about premature deaths amongst its fellow citizens that might possibly be connected with the UK's membership of the EU.