One small molecule, ozone, has both a very good and very bad reputation. On the one hand, we worry about areas in the stratosphere with ozone layer thinning, known as ozone holes. On the other hand, even trace amounts of ozone in our ground-level breathing zone are harmful to our health. To compound these mixed messages, we also hear about the odor-removing benefits of ozone indoors, yet air-cleaning devices that release ozone as a byproduct are harmful.

How can one molecule have so many actions? To make sense of this, let’s look more closely at the structure and chemical reactivity of ozone.

The trioxygen ozone molecule, O3, consists of three oxygen atoms and looks very similar to the dioxygen oxygen molecule, O2, which makes up 20% of the air we breathe. Despite this similarity, the third oxygen atom in ozone, like many threesomes, creates molecular excitability and high reactivity. Unfortunately, at ambient temperatures, ozone does not convert to the life-giving dioxygen gas we breathe.

Ozone has two “extra,” or unbonded, electrically charged electrons hovering nearby, creating what is popularly known as a free radical. Free radicals bond to molecules with opposite charges in the relentless quest for neutral, low energy states.

The neutralizing ozone reactions can impact our health both directly and indirectly through the formation of secondary harmful chemicals, such as aldehydes.

Ground-Level Ozone and Human Health

Ozone is fairly insoluble in water, so when inhaled, our upper airway mucus does not quickly trap it. Instead, the active molecule can travel into the depths of our lungs, where it reacts with biological molecules in our lower respiratory tract. Ozone is especially drawn to thiol and amine groups and double carbon-carbon bonds. For example, thiol, or sulfur-containing molecules, are ubiquitous in our body, contributing to protein folding, enzyme activity, and proper development of stem cells. In short, we do not want these key processes altered.

Another effect of ozone inhalation is the widespread release of free radicals that damage nearby cells through inflammation. Furthermore, ozone disrupts the protective cell-cell junctions between lung cells, creating “leaky lungs,” which allow particles and microbes to move directly into the circulation. Once in the blood stream, particles can travel throughout the body to almost every organ.

Figure 1. Ozone in the stratosphere and at ground level. Image courtesy of Building4Health

How quickly does inhaled ozone cause problems to our health? Unfortunately, even trace amounts that we cannot smell cause harm in less than six hours. The early symptoms include throat irritation, bronchospasm (asthma symptoms), coughing, and inability to inhale deeply without pain. Over time, elevated ozone concentrations are associated with increased asthma attacks, hospital admissions, and deaths from respiratory and circulatory disease.  

Where does ground-level ozone come from? Most outdoor ozone is the result of reactions between volatile organic compounds (VOCs) and combustion products, such as nitrogen dioxide.

Indoors, ozone is released by electronic equipment, photocatalytic air cleaners, electrical circuits, and reactions between VOCs and combustion gases. In some buildings, indoor sources exceed outdoor sources. Indoor mitigation can be achieved by proper ventilation, minimal use of ozone generating appliances, and the presence of mid-range relative humidity. Indoor water vapor (humidity) increases the settling out of ozone from the airborne environment, thus decreasing inhalation doses.

In Summary

  • Ozone is a known outdoor pollutant that is regulated to protect against adverse health effects; and
  • Indoor spaces can either decrease or exacerbate ozone exposure, depending on outdoor infiltration rates; indoor generation of ozone; and, when necessary, ozone mitigating strategies, such as maintenance of mid-range relative humidity (RH 40%-60%), proper ventilation, and the use of charcoal filters in mechanical systems.

We need to be mindful of ozone and limit our indoor exposure to this excitable molecule.