For many weeks now, the debate about the dangers of diesel engines and the threat of diesel driving bans in Germany has been ongoing. In February of this year, some lung doctors once again fuelled the already heated discussion with their doubts about the existing limit values. Although this criticism has since proved untenable, public interest in nitrogen oxides is not abating. For example, comedian Mario Barth repeated his scepticism about the internationally set limit values in his latest programme "Mario Barth deckt auf" - in which our air-Q was also used. One day earlier, the satirical programme "Die Anstalt" also addressed the topic of nitrogen oxide limits and refuted the limit sceptics.
While many diesel drivers now fear that their individual mobility will be restricted, health experts recognise the steadily increasing air pollution as a growing danger to our well-being. In mid-February 2019, the World Health Organisation (WHO) invited experts to Geneva to discuss the health risks of air pollution and possible countermeasures. The focus of the discourse is no longer solely on outdoor air pollution. Indoor air quality is becoming increasingly important in this context. Besides particulatess and volatile organic substances, nitrogen oxides in particular offer a concrete reason for a more in-depth discussion.
What are nitrogen oxides?
There are up to seven different nitrogen oxides in the outside air. Compounds of nitrogen and oxygen to nitrogen oxide compounds are formed exclusively in endothermic reactions, i.e. only when energy is supplied from outside. As a mostly undesirable by-product of various combustion processes, the formation of nitrogen oxides can be attributed to both natural and man-made (anthropogenic) causes.
The natural occurrence of nitrogen oxides in the earth's atmosphere can primarily be traced back to the occurrence of lightning. Both the frequency and the length of lightning are decisive for the nitrogen concentration in the middle and upper troposphere. According to various studies, lightning is responsible for up to 90 % of the nitrogen oxides in this part of the atmosphere.
In addition to fuel combustion in road traffic, the largest emitters of nitrogen oxides in the lower troposphere are combustion plants for fossil fuels such as coal, oil, natural gas and wood, as well as for waste. In closed rooms, it is not only the air entering from outside that causes nitrogen oxide pollution. Furthermore, the concentration of nitrogen oxides in the air we breathe increases due to the combustion of fossil fuels, for example when cooking or when heating fireplaces and coal stoves. The formation of the greenhouse gas nitrous oxide, known as laughing gas, is also attributed to anthropogenic causes. Nitrous oxide is not only produced by the combustion of fossil raw materials. In particular, fertilisation with nitrogenous fertilisers in agriculture contributes decisively to the formation of nitrous oxide (N₂O). However, the nitrogen oxides most relevant for air quality are nitrogen monoxide (NO) and nitrogen dioxide (NO₂). In densely populated urban regions, the combined concentration of the two gases can often exceed 500 µg/m³, with nitrogen monoxide oxidising in the air to nitrogen dioxide. Furthermore, nitrous acid (HNO₂) can be formed by the reaction of NO₂ with water, which is also an environmental pollutant. In addition, nitrogen dioxide is considered a precursor for numerous secondary pollutants, such as nitric acid or inorganic aerosols and photooxidants (e.g. ozone).
Why do we need limit values for nitrogen oxides?
This man-made nitrogen oxide pollution has serious consequences for the ecosystem. Nitrogen oxides are involved in the formation of ozone and smog in the atmosphere. In addition, the nitric acid formed from the reaction of nitrogen oxides, for example, contributes significantly to the formation of acid rain. Nitrogen dioxide in particular is partly responsible for the overfertilisation and acidification of soils and, to some extent, of water bodies. Furthermore, nitrogen dioxide can lead to stunted growth, premature ageing and necrosis (i.e. local death of tissue) in plants.
Since 2004, numerous studies have been published showing a link between daily variations in nitrogen dioxide concentrations and changes in the incidence of respiratory symptoms, hospital admissions and even mortality. Other studies also show a link between disease and mortality rates and long-term exposure to nitrogen dioxide. All short- and long-term studies published so far confirm these harmful effects of nitrogen dioxide already at concentrations on the edge or even below the EU limit values. This limit value is based on the recommendations of the WHO, which has set an annual limit value of 40 µg/m³ and a 1-hour limit value of 200 µg/m³. Evidence from toxicological studies and indoor studies suggests a causal interpretation between nitrogen dioxide and respiratory effects. Therefore, the World Health Organisation is considering adapting its recommendations of limit values for nitrogen dioxide accordingly. In the course of this, the WHO demands, on the one hand, a short-term and epidemiologically justified guideline. On the other hand, it demands a guideline for annual average values based on current knowledge.
Some of the studies were conducted indoors, where nitrogen dioxide is deliberately produced by unventilated combustion appliances. Other studies were devoted to outdoor air, where nitrogen dioxide is merely a component of the air mixture. This complex interrelationship of the individual air components makes the interpretation of those studies difficult, as the causes of verifiable health effects cannot be clearly attributed to NO₂. For this reason, most conclusions about the adverse health effects of nitrogen dioxide are drawn from indoor toxicological and observational studies.
Nevertheless, studies on the quality of ambient air can also provide information on the harmful effects of nitrogen oxides on health. For example, harmful effects of nitrogen dioxide could be demonstrated at specified exposure rates to other pollutants. A change in the effects of particulatess due to nitrogen dioxide could also be proven.
Harmful effects of nitrogen dioxide on health
Exposure to nitrogen dioxide can result in various physical consequences. These include:
- Damage to the mucosal tissues
- Irritation of the eyes
- Damage to the entire respiratory tract
- Increased risk of cardiovascular disease
- Increased risk of type II diabetes
- Increase in mortality as well as cause-specific hospital admissions in the total population
Since nitrogen dioxide has a comparatively low water solubility, it is not bound in the upper respiratory tract, i.e. the mouth and throat as well as the nose and sinuses. Consequently, the pollutant reaches the lower respiratory tract such as bronchi and alveoli and can cause inflammation and damage to cell tissue there. In the bronchial tubes, NO₂ can cause hypersensitivity, which favours the development of allergic diseases of the respiratory tract.
The steady increase in allergic complaints, especially in polluted urban areas, suggests a link between nitrogen oxide exposure and allergy-related immune reactions. Several studies have now concluded that nitrogen oxides in the air we breathe dock onto allergenic proteins. These nitrated allergens presumably promote the production of antibodies that release histamine and histamine-like substances for immune defense, thus causing allergic reactions. Nitrogen oxides can increase the allergy potential of pollen and fine dusts, especially due to air pollution in traffic-polluted urban areas. Humid summer smog can increase the rate of nitration even further, enriching airborne proteins with nitrogen within just a few hours. Therefore, there is a justified suspicion that nitrogen oxide pollution of the air promotes the development of allergies and intensifies the reactions in allergy patients. The typical associated symptoms include allergic rhinitis (hay fever), bronchial asthma, atopic eczema (neurodermatitis), anaphylactic shock, hives and sinusitis.
Between 2014 and 2016, both the World Health Organization, the Swiss Tropical and Public Health Institute (Swiss TPH), and the health authorities of Canada (Canada Health) and the USA (U.S. EPA) were able to uncover associations between exposure to nitrogen dioxide and an increase in all-cause mortality and mortality caused by cardiovascular and respiratory diseases in short-term studies. For the year 2014, at a lower quantification limit of 10 µg/m³ NO₂ over a longer period of time, a total of 5,966 premature deaths were attributed to exposure to nitrogen dioxide. In this context, the years of life lost worldwide were estimated at a total of 49,726.
The increase in cardiopulmonary emergencies, i.e. those affecting the heart and lungs at the same time, as well as the increase in hospital stays due to complaints of the cardiovascular system and the respiratory tract were also associated with an increase in nitrogen dioxide levels.
There is now moderate evidence for the risk of developing diabetes mellitus due to long-term exposure to nitrogen dioxide. In this context, evidence refers to those results that are consistently shown in studies and thus indicate a connection between the occurrence of certain diseases and nitrogen dioxide. Whether this evidence is moderate or strong depends on the number of studies conducted. The studies published so far only include diabetic-related nervous disorders and the so-called "diabetic foot" in their observations of diabetes-typical physical impairments. Other conditions caused by diabetes, such as heart muscle disease and nephropathy, were not taken into account. Therefore, an underestimation of NO₂-related disabilities can even be assumed.
In addition to type 2 diabetes, there are other consistently observed health effects of nitrogen dioxide exposure. These include morbidity due to high blood pressure and mortality due to heart failure and ischaemic heart disease (IHD) as well as strokes.
Due to its irritating and suffocating properties, the gas NO₂ is particularly harmful to the respiratory tract. In the meantime, consistent results of numerous studies provide strong evidence that nitrogen dioxide promotes the development of chronic obstructive pulmonary disease (COPD). Other studies have shown a link between nitrogen dioxide exposure and a wide range of respiratory damage. These include - depending on the duration of exposure and the amount of pollutant absorbed - acute pulmonary oedema, inflammation of the bronchioles or asthma-like reactive airway disease. In addition, NO₂ is partly responsible for increased bronchial reactivity and thus for the development of bronchial asthma, as well as for bronchial constriction and an overall increased susceptibility to respiratory infections. Particularly over a longer period of several years, exposure to nitrogen dioxide poses the risk of impaired lung function in adults and lung growth in children. In addition, the risk of lung cancer increases, as does the risk of reduced birth weight in newborns.
Measures against nitrogen dioxide pollution
According to the WHO, 91 % of the world's population now live in places where air pollutants exceed recommended limits. Worldwide, 4.2 million deaths are attributed to outdoor air pollution and another 3.8 million deaths to indoor air pollution. These dramatic figures reveal an urgent need for action to reduce air pollutants in general and nitrogen dioxide in particular. The necessary measures to improve air quality require political regulation but also individual approaches to solutions.
Political measures:
- Extensive changes in municipal and national transport planning: expansion of alternative drive systems, such as electric motors and refuelling with liquefied gases, introduction of environmental zones to limit pollutants in heavily polluted areas, expansion of local public transport and cycling infrastructure, etc.
- Gradual phase-out of an energy supply based on fossil fuels and simultaneous expansion of renewable energies away from combustion processes
- Implementation of sustainable agriculture and forestry
- Another quite controversial measure is the use of so-called cloud seeding, whereby aircraft are used to release special chemicals that accelerate the formation of clouds and bind air pollutants through the precipitation.
Individual measures:
- Extensive and precise real-time measurements with specific sensors to prevent misclassification of pollutants, to better identify the main pollutant sources as well as their peaks such as long-term exposures
- Also avoid combustion processes indoors if possible, e.g. smoking, cooking on gas cookers or barbecuing in the immediate vicinity, etc.
- Use of air purifiers, taking care not to use other pollutants, such as ozone, in filtering the pollutants.
- Rethinking one's own mobility and consumption behaviour: buying seasonal and regional products as much as possible, making or repairing products oneself, shopping in retail stores instead of having goods delivered to one's home, switching to cycling and public transport, etc.
