Can an improved understanding of the connection between allergens and air quality drive more efficient approaches to allergy prevention and management? And just how strong is the link between the two in the first place?
As the 2007 studies Time Trends in Allergic Disorders in the UK and 50 Years of Asthma demonstrate, in the second half of the 20th century, the prevalence of Type I hypersensitivity disorders such as hay fever, eczema and asthma grew substantially among the British public.
Though the exact causes of such conditions are multi-faceted (and even the source of contention among scientists), various studies show that outdoor air pollutants such as particulate matter (PM), ozone (O3) and nitrogen oxides (NOX) can have an adverse effect on human health, as can a number of indoor pollutants.
Indeed, research from King’s College found that poor air quality is linked to as many as 36,000 deaths per year in the UK. This makes air pollution the top environmental risk to public health — the fourth overall threat to public health after cancer, heart disease and obesity.
The problem shows no signs of abating. In October 2018, the United Nations even warned the UK government that it was endangering people’s health by denying their right to clean air. Though the Great Smog of 1952 may feel like ancient history to some, the air above central London is as much as five times above the air pollution limit set by the EU.
As a response, the UK government launched the Clean Air Strategy in 2019, an ambitious plan to clean up the country’s air and save lives. According to the government website, the strategy “will cut the costs of air pollution to society by £1.7 billion every year by 2020, rising to £5.3 billion every year from 2030.” Air pollution is currently estimated to cost the country more than £20 billion per year.
Meanwhile, Allergy UK estimate that over 20% of the UK’s population are affected by one or more allergic disorders, which is one of the highest prevalence rates of allergic conditions in the world.
Outside the UK, the outlook is equally bleak. By 2025, half of the entire EU population are predicted to be affected by chronic allergy diseases. 50 million Americans (roughly 15% of the US population) experience some form of allergy every year. Globally, allergic sensitisation (when the body’s immune response becomes sensitive to foreign proteins in the environment) is present in up to 40% of the human population.
Given that most allergies were extremely rare only a matter of decades ago, all the signs appear to point the finger of blame at modern, urban living. According to the State of the World Allergy Report 2008 by Pawankar et al, the rate of asthma in developed countries increased 50% per decade in the last 40 years of the 20th century.
The rapid development of large cities with high industrial activity has exposed a wider population to a greater diversity of pollutants. As we approach the third decade of the 21st century, allergy and asthma rates are continuing to skyrocket — not only in developed nations but also in rapidly urbanising parts of the world such as the Asia-Pacific region and Brazil.
Though it may seem like a foregone conclusion, evidence of a causal link between poor outdoor air quality and the increase in allergies and asthma is less clear-cut.
Is there a causal link between outdoor air quality and allergy?
A 2011 study by Samoli et al found that the detrimental effects of the primary outdoor pollutants can directly exacerbate asthma and other pre-existing respiratory problems. Further evidence indicates that traffic-related air pollutants (TRAPs), in particular, have been shown to contribute to the emergence of allergies.
The main source of NO2 and PM is fossil fuels, which are combusted by motor vehicles, factories, and power stations — all hallmarks of an urban environment just about anywhere in the world. Even rural settings are culpable, especially as ammonia (NH3) emissions from agriculture can also have an adverse impact on health.
The prevalence of motor vehicles on the road is of particular concern. Diesel exhaust particles (DEPs) from motor vehicles account for most of the airborne PM in the world’s cities because of the increase of cars with diesel engines in industrialised countries. In a 2013 animal study conducted on young mice, DEP exposure was shown to exacerbate allergic sensitisation.
A 2015 study by the University of British Columbia found that a baby’s exposure to air pollution from traffic increases the chances of developing allergies in the first year of life. Researchers say their study did not find a link between a mother’s exposure to pollutants during pregnancy and a child’s risk of developing allergies, however.
While the evidence may suggest that air quality does indeed cause allergies, a statistical spanner in the works means that establishing a causal link is not so cut and dried. In the UK, for example, the prevalence of allergies may have shot up in recent decades, but data from the Department for Food, Environment & Rural Affairs (DEFRA) suggests that air pollution levels have actually gone down.
Some studies indicate that genetics may be the root cause of allergies.
At the QIMR Berghofer Medical Research Institute in Brisbane, Australia, researchers discovered that genetic risk factors for food allergy show significant overlap with those for other allergic diseases, such as asthma and hay fever.
While genes are undoubtedly a key factor, the fact that humans have been carrying similar DNA for thousands of years without being so allergic suggests the environment plays a role.
One by-product of man-made global warming and warmer temperatures is that plants are given more time to grow, flower, and produce pollen grains. And as atmospheric levels of carbon dioxide (CO2) continue to rise, evidence suggests that plants which produce pollen are producing it in greater quantities. Pollen is a major trigger of hay fever, an allergic disease that is thought to affect 10 to 30% of adults and around 40% of children globally.
When pollen combines with the type of air pollution, the symptoms of allergies such as hay fever can get substantially worse. A 2015 study at the Helmholtz Zentrum München in Germany found that pollen of the common ragweed has a higher concentration of allergens when the plant is exposed to nitrogen dioxide exhaust gases.
Scientists involved in the study discovered that this modified pollen may cause more severe or more prolonged allergic reactions.
The pollen from plants exposed to the nitrogen dioxide was also found to a new protein similar to ones found in rubber trees. The protein was not previously recognised as an allergen from ragweed, and its presence may affect those with allergies to fungi or other plants.
As we have seen, there’s clearly a link between air pollution and the symptoms of asthma and allergies. However, no studies have yet identified clear causation between the prevalence of pollutants such as nitrogen dioxide and the onset of such conditions. Indeed, the rapid rise in food allergies across the world cannot be directly linked to air quality.
Modern living may seem culpable, but many factors appear to play a role in increasing the global allergy rate.
What about indoor air quality?
While studies focusing on outdoor air quality point to an exacerbation of allergies rather than a direct cause, the evidence for indoor air quality shows a much clearer causation.
Second-hand smoking, or environmental tobacco smoke (ETS), is the most harmful indoor air pollutant. Studies show that ETS increases the risk of adult-onset asthma, as well as highlighting a causal connection between postnatal exposure to ETS and the development of childhood asthma. What’s more, exposure to cigarette smoke has been shown to reduce the function of Th1 cells that help regulate the body’s immune system.
Aside from ETS, various indoor building materials, new furniture, fresh paint, dust mites, mould spores, and pet dander (the skin cells that animals shed) may also cause allergies.
Evidence also suggests that exposure to carbon monoxide (CO) during infancy increases the risk of developing hay fever. Far from sheltering us from the harmful allergens associated with air pollution and high pollen counts, our own homes can be a breeding ground for such chemicals.
Reducing air pollution to improve quality of life
Though the causative role of outdoor air pollutants in the development of allergic diseases remains controversial, studies suggest that pollutants can directly make asthma and allergy symptoms worse.
For people who already suffer from allergies, this knowledge can prove useful in managing their conditions and reducing the frequency of allergic reactions.
Of course, it’s much easier for individuals to control indoor allergens and improve the air quality inside the home. Keeping surfaces clean and uncluttered can help cut down on house dust mites, as can regular vacuuming, using air purifiers, and changing bedding regularly. Keeping windows and doors closed can also prevent the spread of pollen throughout a building. The Asthma and Allergy Foundation of America (AAFA) provides some good advice on how to improve indoor air quality here.
Improving outdoor air pollution, on the other hand, is inextricably bound to the fight against climate change and requires a unilateral, concerted effort between nations. Studies show that 95% of the global population breathe dangerous air, so the need to build sustainable cities is one of the most pressing public health and environmental issues of our time.
BreatheLife, a global campaign for clean air led by the World Health Organization (WHO), UN Environment and the Climate & Clean Air Coalition, represents a ray of hope for a sustainable future. With 42 major cities taking part, it’s clear that governments are started to taking the public health crises linked to air quality seriously.
The flagship city for the campaign is the Norwegian capital Oslo, which has implemented a strategy to reduce the city’s CO2 emissions by 95% by 2030. Measures to reach this target include encouraging people to take to electric vehicles (with key cost-saving benefits for those who make the switch), giving pedestrians and cyclists precedence over motorists, and making the city centre primarily car-free.
The far-reaching measures taken by Oslo’s officials have caused others to stand up and take notice. In a 2018 Greenpeace report, Oslo was the only city in the analysis to have emissions below both the EU limit and WHO guidelines. It’s little wonder that the city was voted European Green Capital for 2019.
Though it’s too early to say see if the Oslo approach will drastically improve public health outcomes linked to air quality, reducing the amount of traffic-related air pollutants will certainly improve day-to-day life for people living with allergies and asthma. And with health services and the environment at breaking point, Committee on Climate Change (CCC) recommendations that the UK should reach net zero carbon emissions by 2050 could not be more timely.
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