Strong sunlight and hot weather create the perfect melting pot for chemicals, leading to health warnings this week of a "toxic haze" hitting parts of the UK.
As global temperatures continue to rise, so too does the level of ozone, creating a heady haze in the air. But what is ozone, why is it affected by warmer weather and how does it affect human health?
Ozone (O3) is a gas in the atmosphere that can either be beneficial or harmful depending on whether it's in the stratosphere (upper atmosphere) or troposphere (lower atmosphere).
The troposphere extends around six miles from the ground up and forms the air we breathe. The stratosphere extends from the drop off point of six miles to 30 miles. In the upper atmosphere, ozone forms a protective barrier against the ultraviolet radiation emitted by the Sun. This is a natural layer, one that did not arise from human activity, but is slowly being eroded by it.
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It is the "beneficial" ozone layer that is being destroyed by man-made ozone depleting substances (ODS), used in common industrial manufacturing for refrigerators, air conditioners, fire extinguishers and a range of other goods. These substances are also used as solvents for cleaning, are used to manufacture electronic equipment and as agricultural fumigants.
While the upper ozone is being depleted, human pollution has gradually built up a layer of ozone in the lower atmosphere, creating and compounding "harmful ozone". Essentially, this damaging ozone is the main component of airborne smog.
This smog is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. A VOC is a chemical with a high vapour pressure at ordinary room temperature and a low boiling point – a byproduct of evaporation from a solid or liquid substance. VOC and NOx are produced from industrial manufacturing, motor vehicle exhausts, gasoline vapours and chemical solvents. You would have come across them before; VOC and NOx are the pungent fumes of a petrol station, or the overly acidic scent of chemical bleach.
Ozone is not produced directly into the air; it's the reaction of chemicals within the air. And, like any chemical reaction, it requires a catalyst. Ozone in the lower atmosphere comes into being through a chemical reaction driven by heat and sunlight. In warmer weather, this reaction speeds up and becomes more effective – producing more ozone with the available nitrogen oxide and VOC. Higher temperatures also lead to stagnant air, meaning ozone is concentrated in one place as opposed to circling throughout the atmosphere. Think of it like a car engine – you'll get further on a full tank than running on fumes. With enough fuel behind a reaction, more pollutants combine to create a larger quantity of ozone.
However, there is an interesting phenomenon noted by scientists at Harvard University. At around 35 degrees celsius, or mid-90 Fahrenheit, ozone levels may stop rising in tandem with temperature. This process is known as ozone suppression. While it offers some respite in terms of the upward race of ozone levels, it isn't as reassuring as it sounds. It is an indication of a natural breaking point, when the relationship between temperature and ozone is so intense it actually breaks down.
To put it bluntly, while it may be a lovely sunny day, there's a reason the air is so thick. The lower atmosphere – the air we breathe – is literally "cooking".
Ozone has a very specific reaction once inhaled into the human body. It reacts with molecules in the lining of the airways, causing acute inflammation. This is because the chemical bonds of ozone breakdown in different ways to pure oxygen atoms. Ozone is a triatomic molecule, comprised of three oxygen atoms. As a so-called allotrope of oxygen - meaning it can exist in more than one form - it is much less stable than O2. Whereas oxygen is transparent, ozone is a blue-greyish gas. As such, once it breaks down in our airways, it has a very different reaction to oxygen.
The picture above shows a sample of human lung tissue (epithelia) free from ozone irritation. See those tiny rows of cells that look like eyelashes? Those are cilia. They serve as sensors for a wide range of cells in the body. In the lungs, they have evolved to move back and forth in order to remove microbes and bacteria from the respiratory system. The cilia in this picture form uniform, healthy lines. In contrast, the picture below shows lungs exposed to 20ppb (parts per billion) of ozone over a period of four hours. The cilia are noticeably different. Some are misshapen, others missing.
This damage manifests in a range of different ways, including impairing our ability to protect against microbes, allergens or other airborne toxins. The natural response of airways when confronted by this ozone irritation is to contract, thus impairing breathing. The immediate symptoms of this can be tightness of the chest, pain when breathing, wheezing and nausea. Short-term exposure may be reversible but longer exposure has been indicated as having more significant health repercussions.
In urban areas, higher exposure to ozone has been found to scar the lungs of animals, causing long-term impairment of respiratory function. Studies also suggest it may impair the effectiveness of the immune system.
A report released by the European Society of Cardiology today has indicated that, as the inhalation of PM2.5 pollution increases, the human heart grows larger, causing blood vessels to narrow and increasing the risk of heart failure. In the UK, heart attack and stroke account for eight in 10 of all deaths linked to air pollution.
Ultimately, ozone exposure is deceptive. Initial symptoms can disappear but this doesn't necessarily mean the ozone has stopped doing damage to lung tissue. The most vulnerable groups are children and the elderly, or those with respiratory conditions.
But the problem with pollution is one of human health - and human rights. Given that pollution is clearly linked to harmful health effects, the question remains, are people not entitled to clean air? And if so, how can we counteract pollution?
Since ozone is a part of pollution, the most direct way of combating dirty air is to tackle the industries and cultural bodies that produce the most emissions. The UK is currently in violation of air pollution limits created by the European Commission. The annual limit for pollution in 2017 was exceeded in January. It was only thanks to a court battle with the EU that the UK government revealed new strategies for reducing nitrogen oxide levels – and these have been criticised as woefully ineffective.
Poor air quality has been termed by DEFRA as "the largest environmental risk to public health in the UK", but the latest plans to try to combat this pollution place the responsibility on local authorities. To engage in ozone reduction on a larger level, a structured national plan must come into play, encouraging some to question whether it's time the UK enacted a new Clean Air Act to demonstrate the scale of the problem.
This article was originally published by WIRED UK
