According to the Umwelt Bundesamt, the average annual limit for the NO₂ concentration is set to 40 µg/m³ with the intention to protect people with asthma, small children and older people, especially those with damaged airways. This is lower than the concentration in a lot of German cities and as a result there are discussions about banning driving of diesel vehicles in some German cities.

On the other hand, Deutschlandfunk writes:

Rein wissenschaftlich lässt sich der Grenzwert für Stickstoffdioxid, kurz NO2, nicht begründen. [Scientifically, the threshold for nitrogen dioxide, NO₂ for short, can't be justified.]

Who's right? Is there strong scientific backing for the threshold as a means to help people who's breathing is impaired?

This question has an open bounty worth +50 reputation from Christian ending in 2 days.

Looking for an answer drawing from credible and/or official sources.

I want a well-researched answer to the core question that makes it's case based on referencing empiric arguments based on peer reviewed papers instead of authorities

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    This seems likely to be opinion-based. If the downsides of NO₂ increase as the concentration increases, there will be some point where people draw the line and say the costs outweigh the benefits. Where that line is opinion-based. That said, I understand the estimates of the dangers have been revised downwards recently, and so I would like to see a question about that. – Oddthinking Jan 13 at 11:11
  • @Oddthinking : Feel free to change the question if you see a way to move it in the right direction – Christian Jan 13 at 11:27
  • Some more context might be helpful for those not reading German (is this to combat climate change? is it for public health? etc). Also, should "this is higher" in your second sentence be "lower"? I can't parse how it would imply banning diesels otherwise. – Chris Hayes Jan 13 at 23:28
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    The problem here is that when setting a threshold, they will always set it far lower than where injury starts to appear in order to have a margin. – MichaelK Jan 14 at 9:00
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    I am mystified. The Umweltbundesamt article you linked explains at length exactly which studies were taken into account, including an extensive list of sources. You might argue about the conclusions made based on those studies, but the studies themselves are referenced right there. Which makes me wonder what, exactly, your question is about. Or how you define "strong evidence". Biology and medicine aren't physics and chemistry. You can't find the "right" limit emprically. – DevSolar Jan 14 at 9:17


The question as asked in the title depends on what you consider "strong evidence". I will therefore have a look at which side makes the better-sourced point. (Which is not just about "protecting people with damaged airways", but several other health effects as well.)

One side -- the Umweltbundesamt -- gave references to studies the decision was based upon, and also discusses openly the uncertainties and compromises involved, and dissenting voices.

The UBA freely admits that there is little data on the effects of NO2 in isolation from other traffic-related pollutants. Since we don't allow human experimentation, such data would be hard to come by. But the UBA / EU limits are based in statistical data, which does show correlation, and considers causation at the very least "probable" (EPA).

The other side -- Prof. Dr. em. Köhler -- gave his personal opinion. No more.

The Umweltbundesamt (German Environment Agency) article linked in the question has an extensive list of sources underlining their summaries. The most interesting sources, and the summaries made by the UBA (translations mine, parts where I am not sure about the nomenclature marked with (?)):

  • WHO – World Health Organization (2013a): Review of evidence on health aspects of air pollution – REVIHAAP Project. Technical Report. Copenhagen, Denmark.

...since 2004 numerous epidemiological studies published that show correlations between NO2 short-time exposure and mortality, hospital visits, and respiratory symptoms. The results of these studies were reproducible in chamber experiments (?) and hint that a new recommendation for the short-term NO2 limit might have to be set lower than the current limit. The WHO brain trust recommended an intense review of scientific works on long term exposure, but pointed out that the effects of NO2 could so far not be completely separated from those of other traffic-related air pollutants.

Emphasis in the summary is mine:

WHO committees consulted current European population surveys studying correlations between adult mortality and yearly average NO2 concentrations of below 40 µg/m3. They concluded that health impacts of NO2 have to be accounted for starting from a long-term average exposure of 20 µg/m3.

The effects of long-term exposition to these low NO2 concentrations is discussed controversially among scientists. In 2016, the US Environment Agency (EPA) published a systematic overview of NO2 studies up to 2014, with special focus of separating the effects of NO2 from other pollutants. The correlation between short-term NO2 exposure and asthma attacks is considered to be causal. The correlation between long-term NO2 exposure and the onset (?) of asthma is considered "possibly" causal. In the eyes of the US EPA, there are not enough significant studies showing an effect independent of other pollutants. The same is true, according to EPA, for the studied effects of cardiovascular diseases, diabetes, reduced pre-natal growth, cancer, and mortality.

The UBA then adds its own conclusion:

Independent from this discussion about the significance of the long-term exposure to NO2, which is somewhat controversial in Germany too, it remains certain that traffic related pollutants have detrimental health effects, regardless whether these are caused by NO2 in isolation or in combination with other pollutants. NO2 is and remains a meaningful indicator for traffic induced pollution. For this reason alone, the limit for yearly NO2 exposure serves its purpose for public health care.

Prof. Dr. emeritus Dieter Köhler

This gentleman has gone on record with his views about pollution and the invalidity of current exposure limits on multiple occasions and in various media.

Article, attribution unclear:

The 40 microgram recommendation of the EPA referred to indoor values and the assumption that suspectible persons like infants would breathe this air all year long. The WHO took the 40 microgram recommendation and without further ado made it the limit for outdoors.

This makes it sound as if outdoor limits should not be as low as indoor limits... which is evidently nonsense. For comparison, the limit for workplace exposure in Germany is 950 µg/m³. Indoor conditions can lead to much higher NO2 concentrations, while it is pretty hard to picture how inside concentrations could be any lower than the ones outside. This statement in the second paragraph makes me look askance at the whole article from the get-go.

Köhler is quoted:

"They just made a city-country comparison, found minimal differences in life expectancy. There are of course many, many other reasons why people in the country live a bit, a couple hours longer."

I leave the evaluation of that statement to the reader.


"NO2 is especially harmless, because it is formed in the body itself, from NO. The organism is used to dealing with it. It's reducing it."

Our bodies form CO2 as well, that doesn't mean we can tolerate arbitrary concentrations of it in the surrounding atmosphere. The same is true for many other things our bodies (attempt to) get rid of because they are not healthy. Our bodies expend energy trying to get the concentrations lower. That should be a hint.

The quote continues:

"And it is not traceable why the little bit of NO2 you inhale should result in diabetes."

Please excuse a bit of personal opinion at this point. This is like listening to one of Mr. Trump's speeches. "I don't understand it, I don't like what it would mean to me, so it must be untrue."

All we have to go by are studies. These studies are correlating NO2 exposure with detrimental health effects. Admittedly these studies also correlate a number of other variables with these health effects, because we don't allow human experimentation and NO2 is hard to isolate from those other variables.

But just because we don't know how the mechanism from pollutant to health effect works exactly does not mean it doesn't happen.

And what qualifies as "a little bit" (i.e. negligible even in the long run) and what is a significant amount is the very issue at hand.

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    Currently, this answer just refers to the WHO but not to any specific studies published in peer-reviewed journals. It feels more like it's making arguments by authority then arguing based on empiric evidence. It also not clear to me why the answer ignores animal models as a role for establishing causality. – Christian Jan 14 at 13:15
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    @Christian: You asked about the scientific basis for the UBA recommendation. I pointed out that the UBA referenced its sources well. I will not engage in a meta-study on the state of NO2 research; indeed I would think there are specific stacks for that kind of question with more qualified contributors. -- Is the limit backed by science? Definitely. Could the science be even stronger, should the limit be higher or lower (or the current pollution be measured differently)? Off-topic IMHO. Is Mr. Köhler backing his claims with anything other than his title, at least in the linked article? No. – DevSolar Jan 14 at 13:26
  • -1 for not addressing the claim about smokers. If true that claim in itself would be sufficient to invalidate the weak correlation indicated by the studies. Do smokers inhale enough NO2 to die within months according to the correlations in the studies? – Syren Baran Jan 14 at 18:04
  • @SyrenBaran: Turn it around: People smoke, and don't drop dead. Does that mean that we should not legislate pollution, and un-ban smoking in public places? Besides, that is not the claim asked about in the question -- that was whether the pollution limit is backed by scientific evidence, not whether smokers are a good counter-example. – DevSolar Jan 14 at 18:28
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    @SyrenBaran: So you feel qualified to dismiss a metastudy listing 84 pages of sources, based on thinking you (and / or Mr. Köhler) have found one contradiction in one of the cited studies -- which has been peer-reviewed and published in the Journal of Exposure Science & Environmental Epidemiology in 2007? And expect me to reply to your one-line criticism with scientific reasoning? Sorry, but no. I can live with that -1. If you feel you're on to something, feel free to do a study of your own and publish your criticism. That's how peer-review works. – DevSolar Jan 15 at 15:55

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