COVID-19: Routes of Transmission

Published on 8 March 2021

The COVID-19 pandemic is swinging a wrecking ball through the hyperconnected 21st-century world.  Worldwide deaths due to the virus – SARS-CoV-2 – is 2.5 million as of February 2021.

This virus is taking many paths to invade the human body. Nearly 60 percent of human diseases have a zoonotic origin. Pathogens emerge in places where there is a dense population and the landscape is altered by agriculture, infrastructure development, and deforestation.  In close contact, these infectious agents spill over to humans causing diseases.  Ecological integrity is threatened by encroaching into natural habitats, poaching and trafficking wild animals, as well as, a practice of monoculture.

Fomites are inanimate objects like clothes, furniture, utensils, and other high-touch surfaces, which can transfer infectious microorganisms – virus, bacteria, fungi – to a new host.  With the rise of indoor living, fomites are a threat to human well-being.

How small is a virus?

A micrometer (μm) or micron is one-millionth of a meter. A nanometer (nm) is one-billionth of a meter or one 1000th of a micrometer.

Coronavirus ranges in size from 60 to 140 nm or 0.06 to 0.14 micron. SARS-CoV-2 has a diameter range of 80 to 120 nanometers or 0.08 to 0.12 micron.

For reference:

Average human hair is 50 to 75 microns in diameter.

Average size of human cell is 100 microns in diameter.

Red blood cell is 10 microns.

Bacteria are 1 to 2 microns.

Water cluster size spans 0.1 to 0.3 micron.

Measles virus is 0.22 micron in diameter.

Hepatitis virus is 0.045 micron in diameter.

The diameter of a poliovirus is 0.03 micron, which is 10,000 times smaller than a grain of salt!

Particulate Matter (PM):

PM0.3 (exhaust, heavy metals, fumes) is 0.3 micron.

PM2.5 (ash, soot) is 2.5 microns.

PM10 (dust or pollen) is 10 microns.

Respiratory droplets are 5 to 10 microns in diameter. Aerosols are less than 5 microns in diameter and can be transmitted over distances greater than one meter.

By a guestimate, there are 7 quintillion 5 quadrillion (or 7.5 billion billion) sand grains on earth and 70 sextillion (70 thousand million million million) stars in the observable universe.  Up to 100,000 microbes can live on a grain of sand!  As for the number of viruses on earth, it is 10 nonillion, go figure!

As for the respiratory droplets, large droplets measure 100 to 1000 microns and small droplets measure 1 to 10 microns. Larger droplets measuring 500 microns did not travel long and fell to the ground in about one second. It is estimated that smaller droplets measuring 5 microns can remain in the air for 9 minutes before settling on the ground.

Of note, nearly 5000 coronavirus strains are waiting to be discovered in bats globally.

Here’s a visualization tool to grasp the scale of things from the smallest – Planck Length – to the largest – diameter of the observable universe:

How does the novel coronavirus spread?


SARS-CoV-2 is transmitted via both aerosols and droplets.  It spreads primarily through respiratory droplets.  An asymptomatic individual by merely breathing and speaking can spread the virus, mainly through the airborne transmission of aerosols.  When a person coughs and sneezes, the respiratory droplets remain suspended in the air as droplet nuclei while the larger ones quickly fall to the ground.  The droplets are expelled when breathing, talking, singing, and shouting.  In indoor environments without exhaust fans and poor ventilation, the droplets can stay afloat and move across,  leading to super-spreading.  In air-conditioned rooms, the droplets can be re-circulated.

Respiratory droplets from an infected person can remain on surfaces like handrails, tabletops, countertops, elevator buttons, doorknobs, cell phones, bathroom fixtures and toilets, bank ATMs, supermarket self-checkouts, subway trains, and airport check-in kiosks.

Respiratory droplets may contain 7 million virus particles per milliliter.

A single cough can release 3000 droplets, which can travel up to 80 kilometers per hour.

A single sneeze can release 30,000 droplets, which can travel up to 320 kilometers per hour.

More than 1000 virus-containing droplets are released during a minute of loud speech.

A single breath can release 50 to 5000 droplets.

An infected person in a single cough or sneeze can release 200 million virus particles.

Aerosols are smaller, lighter, contain less virus than respiratory droplets, and in the absence of fresh air, they can linger in the air for a longer period.

As aerosols, SARS-CoV-2 can remain in the air for at least 3 hours and on surfaces for several days, but the viability diminishes over time.  During invasive procedures like the intubation of an infected patient in a hospital setting, aerosols are generated.  Toilet flushing does aerosolize the droplets.  SARS-CoV-2 genetic material was found on toilets used by COVID-19 patients.  The discovery of coronavirus in the bathroom of an unoccupied apartment, which was directly above the home of five people with confirmed COVID-19 in Guangzhou, China, suggests that the airborne pathogen can spread via drain pipes.

Infection can be acquired through shared toilets in airplanes used by asymptomatic passengers. In the case of air travel, most modern aircraft use 50 percent fresh air and 50 percent recycled air filtered through HIPAA filters, similar to the ones used in surgical operating theatres.

In well-ventilated places, the viral load is lower.  In crowded, poorly-ventilated places, near an infected person’s toilet in a hospital setting or a dwelling, and places where healthcare workers remove their personal protective equipment (PPE), the viral load is higher.

Hotspots in hospitals include COVID-19 isolation wards, hand-sanitizer dispensers, hand gloves doorknobs, and most commonly used medical equipment like self-service printers, computer keyboards, desktops, cell phones, and telephones.

COVID-19 patients exhaled coronavirus into the air at an estimated rate of 1000 to 1,00,000 RNA copies per minute.  They can even exhale millions of virus particles per hour.  This was mainly influenced by the disease stage, patient activity, and age.  During earlier stages of COVID-19, it was one lakh viruses per minute, spread sporadically.  Contact with a contaminated surface is the second most common route of virus transmission, according to World Health Organization (WHO).

How long does coronavirus remain on surfaces?

Viruses are mostly found on flat and hard surfaces and not so much on rough surfaces.  The virus survived on nonporous smooth surfaces for longer than porous surfaces like cotton.

The coronavirus can only survive for 4 hours on copper.

On printing and tissue papers for 3 hours.

On cardboard for 1 day.

On cloth and treated wood for 2 days.

On glass and banknotes for 3 to 4 days.

On stainless steel for 2 to 7 days.

On plastic for 3 to 6 days.

On outer layer of surgical masks for 7 days.

A resilient SARS-CoV-2 could survive on smooth surfaces like glass on phone screens, stainless steel, and banknotes for 28 days!  It can survive longer in cooler weather.  At 40 degree Celsius, the virus survival declines.  The virus survives at room temperature of 20 to 25 degree Celsius, humidity of 40% to 50% and pH range of 3 to 10.  Laboratory conditions conducive to coronavirus survival include 4 degree Celsius and 20-80% relative humidity. An Australian study has investigated how different temperatures impact the reduction in the amount of viruses on surfaces.

It is important to keep in mind that some of these findings are in a carefully controlled laboratory setting and may differ in the real world due to fluctuating patterns of temperature, humidity, and light.

Can the use of masks help?

Source control is crucial and using a mask helps greatly, as mask-wearing is an effective way to limit transmission of the virus from an infected person.  Masks reduce hand-to-face contact.  Single-use masks when disposed of incorrectly can spread the virus.

Cloth mask can filter particles of size 10 microns.

Surgical mask can filter particles ranging in size from 2.5 to 7 microns.

FFP1 mask can filter particles of size 0.6 microns.

N95 and N99 masks can filter particles ranging in size from 0.3 to 0.5 microns.

UV mask can filter particles of size 0.1 microns.

For a face mask to be effective, it needs to be well sealed and dense enough to filter particles as tiny as 0.1 micron.  Although mostly similar, each country has its own standard for the types of masks.  There are single-use face masks, surgical masks, and respirators.  Advanced filtration systems such as N95 (FFP2), N99 (FFP3) and N100 are capable of filtering particles which are 0.3 micron or larger in diameter.  India follows BIS, ISO certification for masks.  For respirator masks, China follows the KN standard.  US follows the N standard.  Europe follows filtering face piece (FFP) score derived from EN standard.  3M is a company that produces masks that follow KN and N standards.  Air purifiers use high-efficiency particulate air (HEPA) filters.  Ultraviolet (UV) mask uses UV-C light with dual-way filters for sterilizing and sanitizing air in real-time.  It filters particulate matter except for oil-based particles or chemical vapors.

A manual for making home-made masks was issued by the Office of the Principal Scientific Advisor to the Government of India in March, 2020.

Of note, the Japanese have been sporting masks since the 17th century.  As for the Jains of India, wearing white masks is a part of their holy attire, a practice that dates back to the 5th century BCE!

Can a breach in PPE or lack of it lead to infection in a healthcare setting?

When there is a perforation, pinhole, or any type of breach in personal protective equipment (PPE), there is a potential for microbes to penetrate and infect the healthcare worker.  The glove hole ranges in size from 30 to 50 micron.  Double gloving, changing gloves frequently, and correct application and removal of gloves can help.

According to a recent study, there was a case of proven transmission of SARS-CoV-2 from a lung donor to a recipient, although the donor tested negative for the virus through RT-PCR on a nasopharyngeal (NP) swab test obtained within 48 hours of procurement.  The recipient developed fever, low blood pressure, and pneumonia on the third-day post-transplant.  While the RT-PCR test on an NP swab was negative, the bronchoalveolar lavage (BAL) fluid was positive for the virus.  One of the surgeons who was only wearing a surgical mask and not the PPE or N95 mask while preparing the lungs for transplant was also infected with COVID-19.

What are super-spreader events and risky jobs?

According to Japanese research, only a small proportion of people who are infected with COVID-19 infect others.  This is called a super-spreading event or cluster.  Only such clusters can transmit and sustain the novel coronavirus.  About 70 to 80 percent of the infected people do not infect others.  A cluster-based approach is quite useful to suppress virus transmission.

Super-spreader events and virus-preserving environments include weddings, funerals, birthdays, schools, cruise ships, nursing homes, prisons, karaoke bars and clubs, restaurants, movie theatres, places of worship, choir, gyms, call centres, indoor sports, meatpacking and processing plants, and cold storage facilities.  Research shows that speaking loudly, shouting, and singing can increase the risk of spreading the coronavirus contagion.  Speaking generates droplets which evaporate, leaving behind aerosols with viable virus.

In the slaughterhouses where fresh meat and fish are processed, the virus can survive in blood and oils.  The virus is found to survive in frozen foods, but its viability is not clearly understood.

A study at the Max Planck Institute for Evolutionary Anthropology in Germany has found that people with a cluster of genes on chromosome 3 inherited from Neanderthals who lived more than 50,000 years ago have a 60 percent higher chance of contracting severe COVID-19.  It is almost non-existent in people from Africa and East Asia.  It is present in 16 percent of Europeans and 9 percent of admixed Americans.  More than half the population of Bangladesh carries at least one copy of these genes.

As of March 2021, SARS-CoV-2 in Bengaluru, India, was found to be mutating faster. Next-generation sequencing (NGS) and mass spectrometry on nasopharyngeal swabs of COVID-19 patients detected 27 mutations. Of note, the phylogenetic analysis of the virus indicated that it was closely related to the isolates from Bangladesh. This research shows that the Indian isolates have multiple origins.

Can SARS-CoV-2 spread to humans from frozen foods?

Investigations by both the World Health Organization (WHO) and the Chinese authorities into the origin of SARS-CoV-2 speculate that the virus made the jump to humans from frozen foods.  It is understood that viruses can survive in frozen environments.  Frozen animals are sold in Chinese wet markets.  According to research, the virus can persist on frozen food packaging, but it is not known if it can survive in its infectious form.

SARS-CoV-2 is an enveloped virus with a fatty lipid membrane that it uses to infect human cells. This lipid membrane is vulnerable to cycles of freezing and thawing, which the frozen foods undergo during transit.  Survival of the virus under freezing temperatures of 10 to 20 degree Celsius is not well understood.  While in airplanes, the temperature can drop down to minus 30 degree Celsius in cargo holds which store frozen foods, in ships the salt content of air can disrupt the virus.  Changes in humidity can also impact the virus.

The RT-PCR test, which is used to detect the presence of the coronavirus, does not differentiate between viable and non-viable viruses.  Also, cooking kills the virus, but when food is raw or not cooked properly, it has the potential to spread the virus.

Was COVID-19 detected earlier outside China?

According to a study, COVID-19 infections were found in the US in December 2019.  The epidemic is believed to have started in late January 2020 in France, but RT-PCR done retrospectively on a stored respiratory sample confirmed SARS-CoV-2 infection in a patient who was hospitalised at the end of December 2019. In Italy, the novel coronavirus was circulating as early as September 2019.

What is wastewater epidemiology?

An infected person sheds the virus in bodily fluids like saliva, mucus, and feces, which then end up in sewage.  By sampling RNA particles from wastewater, it is possible to study the viral spread and their mutations.  Wastewater testing is rapid and a whole population can be monitored easily.  The drawbacks also include the mixing of sewage from hospitals, slaughterhouses, and homes, which can skew results.

Can a mosquito spread COVID-19?

A mosquito that bites a person with COVID-19 cannot pass the coronavirus infection to its next victim.

Mosquitoes are notorious disease carriers, transmitting West Nile virus, Zika virus, dengue virus, and Chikungunya virus from person to person and among animals.  While a particular species of mosquito survives only on human blood, some other species prefer birds and reptiles.  Female mosquitoes seek blood meals to extract proteins and iron needed to produce eggs.

How is a cold different from COVID-19 and Flu?

There are more than 200 viruses that can cause a common cold, including the human coronavirus and the respiratory syncytial virus.

Cold symptoms occur gradually.  It includes sneezing, mild cough, aches and pains, sore throat, and runny or stuffy nose.

The onset of flu symptoms is abrupt.  It includes fever, fatigue, dry cough, headaches, and body pain.COVID-19 symptoms include fever, dry cough, sore throat, body pain, fatigue, headaches, and shortness of breath.

Coronaviruses like OC43, 229E, NL63, and HKU1 cause the common cold.

How to mitigate virus transmission during dry, cold weather?

Dry winter air makes the virus stable and enables transmission of the virus.  Inhaling dry air impairs the respiratory tract immune defense.  During winter, people are more likely to stay indoors, which makes virus transmission via droplets much easier.  Ventilation systems will need powerful filters and should not recirculate the air in buildings.  It is now well-known that the virus transmission takes place via both large and small droplets in the air.  There should be a sustained focus on effective ventilation and the use of state-of-the-art air purifiers.

How to safeguard?

Direct sunlight, heat, and humidity impact viral survival; ultraviolet light is a powerful disinfectant. Regular disinfectants kill the virus in five minutes. Maintain six feet distance from an infected person. Speaking softly can reduce the spread of the infection. Establishing quiet zones in high-risk indoor environments can curb the spread. Shared surfaces should be cleaned well; for instance, utensils in a cafeteria. Use appropriate cleaning products on surfaces that may have blood, stool, or body fluids on them. Wear gloves while cleaning and make sure there is good ventilation while using cleaning products. Avoid enclosed spaces with poor air circulation and high density of people for prolonged periods. Windy outdoor spaces will dilute and reduce the viral load. People must be reminded to wear masks and maintain a safe distance. Remember that the amount of virus released from an infected person changes throughout the course of infection and differs from person-to-person.

The most important variable is the air we breathe.

Image Source: Pexels and Unsplash

Further reading:

Detection of SARS-CoV-2 in the air from hospitals and closed rooms occupied by COVID-19 patients (January 4, 2021): “Our results indicate that the chance of picking up SARS-CoV-2 in the air is directly related to a number of COVID positive cases in the room, their symptomatic status, and the duration of exposure and that the demarcation of hospital areas into COVID and non-COVID areas is a successful strategy to prevent cross infections. In neutral environmental conditions, the virus does not seem to spread farther away from the patients, especially if they are asymptomatic, giving an objective evidence for the effectiveness of physical distancing in curbing the spread of the epidemic.”

New CDC Guidelines (April 5, 2021): SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments

Covid-19 has redefined airborne transmission (April 14, 2021): “Improving indoor ventilation and air quality, particularly in healthcare, work, and educational environments, will help all of us to stay safe, now and in the future.”

Ten scientific reasons in support of airborne transmission of SARS-CoV-2 (April 15, 2021): “There is consistent, strong evidence that SARS-CoV-2 spreads by airborne transmission. Although other routes can contribute, we believe that the airborne route is likely to be dominant. The public health community should act accordingly and without further delay.

Higher airborne pollen concentrations correlated with increased SARS-CoV-2 infection rates, as evidenced from 31 countries across the globe (March 23, 2021):As we cannot completely avoid pollen exposure, we suggest wide dissemination of pollen−virus coexposure information to encourage high-risk individuals to wear particle filter masks during high springtime pollen concentrations.”

The Economist (29 May 2021): Improving ventilation will help curb SARS-CoV-2

A guideline to limit indoor airborne transmission of COVID-19 (April 27, 2021):Airborne transmission arises through the inhalation of aerosol droplets exhaled by an infected person and is now thought to be the primary transmission route of COVID-19.

A fresh (air) look at ventilation for COVID-19: Estimating the global energy savings potential of coupling natural ventilation with novel radiant cooling strategies (June 15, 2021):As closed environments, buildings have become sites of rapid COVID-19 transmission. In this research, we demonstrate the energy cost of increasing outdoor air supply with standard systems per COVID-19 recommendations and introduce an alternative HVAC paradigm that maximizes the decoupling of ventilation and thermal control.”

Antibodies to SARS-CoV-2 in All of Us Research Program Participants, January 2-March 18, 2020 (June 15, 2021):Our findings indicate SARS-CoV-2 infections weeks prior to the first recognized cases in 5 US states.”

Smartphone Screen Testing, a novel pre-diagnostic method to identify SARS-CoV-2 infectious individuals (June 22, 2021): “….PoST is a new non-invasive, cost-effective, and easy to implement smartphone-based smart alternative for SARS-CoV-2 testing, which could help to contain COVID-19 outbreaks and identification of variants of concern in the years to come.”

Transmission of COVID-19 and other infectious diseases in public washrooms: A systematic review (27 August 2021): “Open-lid toilet flushing, ineffective handwashing or hand drying, substandard or infrequent surface cleaning, blocked drains, and uncovered rubbish bins can result in widespread bacterial and/or viral contamination in washrooms.”

Researchers developing air quality sensors to detect COVID-19 (3 September 2021)

Mask Wars (23 September 2021)

Efficacy of homemade face masks against human coughs: Insights on penetration, atomization, and aerosolization of cough droplets (14 September 2021): “The current study is directed toward those people who cannot use the recommended N95 mask due to lack of convenience, availability, or economic and demographic reasons. This study recommends the safer option among commonly available fabrics that the said population generally uses for homemade masks without compromising on actual safety against COVID-19.”

Real-world data show that filters clean COVID-causing virus from air (6 October 2021): “An inexpensive type of portable filter efficiently screened SARS-CoV-2 and other disease-causing organisms from hospital air.”

Hand washing and sanitizing not enough: Close that toilet lid after flushing! (3 November 2021): “Leaving toilet lids open after flushing can disperse contaminated droplets beyond a metre and remain in the air for 30 minutes. This is one of the findings revealed in a global review of the risks of bacterial and viral transmission in public bathrooms.”

Two-metre COVID-19 rule is ‘arbitrary measurement’ of safety (23 November 2021): “The researchers say that while the two-metre rule is an effective and easy-to-remember message for the public, it isn’t a mark of safety, given the large number of variables associated with an airborne virus. Vaccination, ventilation and masks – while not 100% effective – are vital for containing the virus.”

Data Review: How many people die from air pollution? (25 November 2021): “According to the authors 5.5 million people die prematurely every year due to air pollution from all anthropogenic sources. This includes the air pollution caused by agriculture, residential energy use, non-fossil industrial emissions, and fossil fuel burning. It is the sum of indoor air and outdoor air pollution that originates from anthropogenic sources.”

An upper bound on one-to-one exposure to infectious human respiratory particles (1 November 2021):Wearing face masks and maintaining social distance are familiar to many people around the world during the ongoing SARS-CoV-2 pandemic. Evidence suggests that these are effective ways to reduce the risk of SARS-CoV-2 infection. However, it is not clear how exactly the risk of infection is affected by wearing a mask during close personal encounters or by social distancing without a mask. Our results show that face masks significantly reduce the risk of SARS-CoV-2 infection compared to social distancing. We find a very low risk of infection when everyone wears a face mask, even if it doesn’t fit perfectly on the face.”

Beijing city urges end to overseas deliveries over Omicron (18 January 2022): “Beijing city officials are recommending people stop ordering items to be delivered from overseas, after saying a local woman may have been infected by Omicron after opening a parcel.”

Aerosols must be included in climate risk assessments (21 November 2022)

Air purifiers: indoor pollution kills but many devices are ineffective and some may even cause harm (15 December 2022)

Eurosurveillance: Europe’s journal on infectious disease surveillance, epidemiology, prevention and control

Last updated on 23 December 2022

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