“There’s never been, in history, so much action about indoor air quality.”
- Lidia Morawaska, Ph.D., physicist and distinguished professor, School of Earth and Atmospheric Sciences, Queensland University of Technology, director of the International Laboratory for Air Quality and Health
“It’s not the size of the dog in the fight, it’s the size of the fight in the dog.”
- Mark Twain
Because indoor air can be full of pathogens such as the flu and coronaviruses, public health officials hope that the COVID-19 pandemic will spur lasting improvements to the air we breathe. In this week’s Germ Gems post, I focus on airborne viruses and also spotlight PM2.5—a submicroscopic particle that is life-threatening and, in addition to being a coconspirator with SARS-CoV-2 in the development of COVID-19, is responsible for the health consequences of air pollution.
A primer on airborne particles. Scientists usually describe airborne particles in microns (or micrometers). One micron is 1/1000 mm or 1/25,000 of an inch. (For reference, the diameter of a human hair is roughly 70 microns.) Particles ranging from 0.1 to 2.5 microns present the greatest health concern because they are small enough to get past the tiny hairs that line our breathing passages but too large to be easily exhaled.
A refresher on airborne transmission of SARS-CoV-2. Scientists define droplet versus aerosol transmission on the basis of size: “droplets” are considered to be emissions larger than 5 or 10 microns in diameter, whereas those smaller than 5 microns are termed “aerosols.” Both droplets and aerosols can be generated during coughing, sneezing, talking, or exhaling. Large droplets, however, settle quickly but small aerosols can remain airborne and may transport over longer distances by airflow.
In the early months of 2020, scientists engaged in extensive research and debate as to whether SARS-CoV-2 was transmitted via droplets or aerosol. The researchers ultimately concluded that SARS-CoV-2 is airborne, that is, it istransmitted from person-to-person mainly by aerosols rather than droplets. The conclusion they reached on this controversial issue resulted in recommendations to help slow the spread of this virus such as social distancing and what type of mask to wear.
The profusion of viruses in air. Viruses are incredibly tiny. (In the case of SARS-CoV-2, for example, it’s suggested that 100 million viral particles [virions] can fit on a pinhead.) For those viruses floating in air, all are waiting an opportunity to enter the respiratory tract either alone or, more commonly, via a vehicle (either an aerosol or droplet).
Fortunately, most of the viruses that cause a respiratory tract infection are just a nuisance, the cause of a “common cold.” But there are others that despite their small size have an outsized economic and health impact, in particular influenza, SARS-CoV-2, and respiratory syncytial virus (RSV). If you seek medical care, your physician will consider these viruses as well as parainfluenza viruses, metapneumovirus, adenovirus, rhinoviruses, and enteroviruses as the cause of your illness.
What you need to know. From a practical standpoint, you need concern yourself mainly with the three airborne viruses that are currently causing a so-called tripledemic—SARS-CoV-2, influenza virus, and RSV.
Emphasis on these three viruses is related to several shared characteristics: (1) they can readily make their way into your lower respiratory tract (that is, your lungs) where they can precipitate major illness or even death; (2) vaccines are available for the prevention of SARS-CoV-2 and influenza (an RSV vaccine is in the offing for adults later this year); and (3) Federal Drug Administration (FDA)-approved medicines are available for outpatient treatment (Paxlovid for COVID-19; oseltamivir [Tamiflu®] or baloxavir [Xofluza®]) for influenza. (Monoclonal antibodies [Palivizumab] are FDA-approved for prevention of severe RSV in high-risk infants).
There is an enormous overlap in the symptoms caused by these three viruses (fever, cough, sore throat, congestion or runny nose, sneezing, loss of smell, nausea or vomiting, diarrhea, fatigue, muscle or body aches, and loss of appetite). Therefore, symptoms are of little value in pointing to a specific etiologic diagnosis. If you have symptoms of a respiratory tract infection and are short of breath, lightheaded, or confused, contact your doctor and let them determine the cause of your infection and the best course of treatment.
So what’s the big deal with PM2.5? Fine particulates, such as, PM2.5, can penetrate deeply into the lungs, where they can irritate and corrode the alveolar lining. They are known to play a critical role in many of the diseases associated with air pollution, including cardiovascular disorders, chronic obstructive lung disease, bronchitis, asthma, and hypertension. PM2.5 can also drive the initiation and progression of diabetes mellitus and cause adverse birth outcomes as well as premature death.
In addition, in April 2020, researchers at the Harvard University T.H. Chan School of Public Health reported that exposure to PM2.5 was associated with higher death rates from COVID-19. (Subsequently, researchers found that PM2.5 serves as a potential SARS-CoV-2 carrier into the airways.) And, in a March 2023 article in MedPage Today, “Exposure to Air Pollution Linked to Risk of Long COVID in Young Adults,” researchers make the case that PM2.5 and SARS-CoV-2 can be conspirators in the development of long Covid, a disabling illness that afflicts an estimated 19 million Americans.
Prevention of viral respiratory tract infections. Vaccines fortify the immune system’s ability to stop progression of infection caused by viruses that find their way into the respiratory tract. As noted above, vaccines are presently available for the prevention of SARS-CoV-2 and influenza. In addition, two key strategies to reduce the initial access of viruses into the respiratory tract are available: air filtration and masks.
Air filtration As early as June 2020, scientists recognized the value of air disinfection (filtration) in preventing SARS-CoV-2 access to the human airway (see Journal American Medical Association, “Airborne Spread of SARS-CoV-2 and a Potential Role for Air Disinfection”). In March 2022, the U.S. government launched a “Clean Air in Buildings Challenge” to spur building owners and operators to improve the ventilation and indoor air quality in their buildings. And in December, the White House announced that all federal buildings would meet minimum air safety requirements.
But huge challenges lie ahead, particularly for air filtration of schools, office buildings, and other public venues. Retrofitting them with the technology to deliver clean air will be an immense and costly undertaking. In the long-run, however, the economic savings from a public health perspective makes it clearly worth it. The process of making indoor spaces safe from infectious agents could also reduce exposure to pollutants, such as, fine particulates like PM2.5.
Masks I covered the issue of masks in my January 19, 2022 Germ Gems post “Mask and Covid-19: The Devil is in the Detail.” After much debate and further research, the bottom line is: "they work." But only certain masks, also called "respirators." The 3M N95 medical-mask effectively filters 95% of particles in the air—including SARS-CoV-2. Medical professionals around the world use this mask and the comparable KN95.
What’s needed now? Clean air (indoor and outdoor) is of paramount importance to our health. What we now need is for scientists from multiple disciplines across the globe, such as, public health, engineering, economics, and political science, to work in harmony with government officials to make lasting improvements to the air we breathe. If this were to happen, that would be a greatly needed “breath of fresh air,” both literally and figuratively.