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Air pollutants: Ozone

What is Ozone?

The Ozone, also known as trioxygen, is a molecule composed of three atoms of oxygen (with the chemical formula O3). It is protecting us from UV radiation through the Ozone Shield (or also Ozone Layer).

ozone layer protection

However, if the ozone is found on surface-level, it can become a harmful gas that can cause serious health issues when inhaled in high concentrations. For this reason, Ozone (O3) is divided into “Good Ozone” (Ozone Shield) and  “Bad Ozone” (Ground-Level Ozone).

“Good” vs “Bad” Ozone

Good Ozone: The ozone layer

Good Ozone is created naturally in the upper atmosphere (at 20 km to 30 km above Earth), reason why it is called stratospheric ozone. It forms the well-known ozone layer, which serves as the Earth’s shield to protect us from solar ultraviolet radiation. Moreover, it is considered one of the primary greenhouse gases in the atmosphere.

Since the beginning of the Industrial Revolution, the emissions of carbon dioxide to the atmosphere have substantially increased. These activities have partially destroyed this protection layer and leaded to the ozone layer depletion, commonly known as “the ozone hole”.

Bad Ozone: Ground-level ozone

Bad Ozone (also named ground-level ozone or tropospheric ozone) is a toxic gas composed of a combination of gases and sunlight. The ozone formation formula is: VOCs + NOx + Heat + Sunlight = O3.

It easily reacts with some hydrocarbons, being the main component for smog, which causes severe sickness or even death.

This ground-level ozone is considered a criteria air pollutant due to its toxicity to humans and the environment. In order to control it and protect population, several air pollution standards involving ozone have been established worldwide.  

Ozone does not only have effects on urban environments, as wind can make it travel long distances, affecting rural areas as well. Furthermore, Ozone can also be formed inside houses by electronic devices. For instance, there are many “air purifiers” which generate ozone as a way to destroy other harmful pollutants. Indoor air pollution can become so dangerous for humans’ health if it is not detected and solved on time. Ozone reacts with organic materials in the same way it does with other gases to create smog, altering the chemical compositions around us.

smog due to ozone

What are the health and environmental effects of ozone?

As it has been said, ground-level ozone can cause a variety of respiratory health effects. It affects on greater way sensitive groups such as elderly, children or people of all ages who have sensitive lungs. However, breathing high concentrations of ozone is very dangerous for everyone.

Short-term exposure can cause a variety of respiratory health effects, including chest pain, coughing, throat irritation, and airway inflammation. It also can reduce lung function and harm lung tissue or decrease the capacity to perform exercise by the shortness of breath. Moreover, ozone has been linked with premature mortality or worsening respiratory illnesses like bronchitis, asthma or emphysema.

On the other side, many studies have proven that long-term ozone exposure can provoke asthma, damage lung tissue for life and increase the risk of death from respiratory causes.

Ozone can significantly damage vegetations and ecosystems. During growing season, when these are more sensitive, it causes photosynthesis reduction, slowing plants’ growth and increasing the risk of disease, insects, heavy weather conditions or other pollutants’ effects. It can be also related with heat waves since plants absorb less ozone and its concentration rises.

heat wave

How is ozone pollution controlled?

Over the years, developed countries have reached some agreements to control pollution. Each country or state has its own implementation plan. For example, the Clean Air Act made by the U.S. Environmental Protection Agency (EPA) establishes National Ambient Air Quality Standards (NAAQS) for those criteria pollutants considered most harmful for health and the environment. Another example is the European Union Air Quality Directive by the European Environmental Agency, which also establishes some standards and tips to reduce air pollution, both indoors and outdoors.

Ozone, especially ground-level ozone, is considered both a primary and secondary criteria pollutant. As already mentioned, it can become a huge issue for the environment and for humans health. Maximum permitted or recommended levels for O3 are:

  • World Health Organization (WHO) guidelines: 0,070 ppm (140 μg/m³) for annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years.
  • NAAQS: In addition to WHO guidelines, in USA ozone is also monitored each hour with a maximum concentration of 0.12 ppm (235 μg/m³).
  • EU Air Quality Directive: maximum daily 8-hour mean of 120 μg/m³, which cannot be exceeded more than 25 days over 3 years.
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