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Ozone is a substance that occurs naturally in a gaseous state, about 10 km above the earth's surface (stratosphere). This ozone is beneficial to our health, as it absorbs UV rays from solar radiation and protects us from their harmful effects. However, it is considered a pollutant when it is at the surface, as it is a reactive and highly oxidative gas with low solubility in water. Exposure to this gas occurs by inhalation, and it is then absorbed into the upper respiratory tract where, because of its low solubility in water, it is not effectively eliminated. Therefore, the vast majority of ozone travels into the intrathoracic airways and is absorbed mainly in the lower airways as it dissolves in the ELF (epithelial lining fluid). The reactions that occur upon dissolution generate secondary oxidation products, which produce oxidative stress that can cause cellular damage and alter cell signaling in the airways.
Although monitoring and regulation are generally applied to outdoor spaces, most inhalation occurs indoors. Indoor ozone concentration depends on several factors including gas exchange rate, ozone loss by reaction with organic gases and, in densely occupied spaces, ozone loss in occupants, among others.
The importance of ozone as an air pollutant has been recognized since the middle of the 20th century. The WHO establishes a limit of 50 ppb maximum daily exposure considering an 8-hour working day. As for the European directive and the Spanish legislation, they refer only to outdoor air quality and determine an exposure limit of 60 ppb, also for an 8-hour average. The daily environmental exposure limit value (ELV-ED) sets these thresholds depending on the work performed during a 40-hour working week: 50 ppb for heavy work, 80 ppb for moderate work and 100 ppb for light work.
The ranges of values used as indicators are based on the values recommended by the WHO and European bodies, and on the levels above which adverse health effects occur.
Ozone concentration can be measured either in µg/m³, ppm or ppb. Although MICA collects the values in ppb, the conversion can be made taking into account the ambient temperature conditions. The relationships between the above mentioned units for a temperature of 20 °C are shown below:
- 1 ppb ~ 0.01 ppm ~ 2 µg/m³
Ozone generally originates outdoors and enters indoor spaces along with ventilated air. Outdoor ozone is found naturally in the stratosphere, and as a secondary pollutant in the earth's surface or troposphere. In the latter case, ozone is formed as a result of chemical reactions in the presence of VOCs and NOx (primary precursors), and solar radiation. In addition, there are also indoor ozone emitting sources. These sources include bleaches, bleaching agents, disinfectants, deodorants, cleaners, microbiocides and water treatment products. Ozone is also frequently emitted internally by office equipment such as laser printers and photocopying machines, or as a by-product of air purification processes with UV filters.
An ozone-free space is beneficial to health. The lower respiratory tract is free of secondary oxidation products from ozone chemical reactions, and the cellular damage they cause, along with inflammation and consequent alterations, is prevented. The absence of ozone is very important to ensure a healthy space in which the respiratory and immune systems, among others, are not attacked.
Short-term elevated ozone concentrations can cause coughing, sore or scratchy throat, as well as difficulty and even discomfort in deep breathing. It can inflame and damage the airways and increase the susceptibility of the lungs to infection by altering the protective barrier function of the epithelium. It can also aggravate respiratory diseases such as asthma, emphysema and chronic bronchitis. Asthmatics may notice an increase in the frequency of asthma attacks.
In addition to the respiratory system, it also affects other systems such as the cardiocirculatory, immune, gastrointestinal and liver systems. Occasionally, it can damage the skin and mucous membranes or even cause neurotoxic effects. As is usually the case, the effects may be aggravated or more prevalent in asthmatic persons and vulnerable risk groups, such as the elderly or infants. In addition, people with certain genetic characteristics and lack of intake of certain nutrients such as vitamin C and E are also more likely to be affected by ozone exposure.
Tips to reduce indoor ozone concentrations:
- Ventilation systems with active carbon filters
- Elimination of pollutant sources
