SARS spreads primarily through respiratory droplets and tiny airborne particles released when an infected person coughs, sneezes, talks, or breathes. Most transmission happens at close range, typically within about 1 meter of an infected person, though smaller particles can travel farther and linger in poorly ventilated spaces. The virus can also survive on surfaces for hours to days and, in rare cases, spread through contaminated plumbing systems.
Respiratory Droplets and Airborne Particles
When someone with SARS exhales, coughs, or sneezes, they release a mix of larger droplets and smaller aerosol particles. Larger droplets (5 micrometers and above) fall to the ground relatively quickly and pose the greatest risk within about 1 to 1.5 meters. Smaller aerosol particles, under 5 micrometers, behave differently. They can hang suspended in the air for extended periods and drift well beyond 1.5 meters, especially indoors.
Indoor transmission typically occurs when two people are within roughly 0.7 to 1 meter of each other. But distance alone doesn’t guarantee safety. Prolonged time in a poorly ventilated enclosed space, even with adequate distancing, has been linked to outbreaks. Activities like talking, singing, eating, and exercising all generate aerosols that accumulate when airflow is stagnant.
When Infected People Are Most Contagious
SARS has a mean incubation period of about 5 days, with a range of 2 to 14 days. During this window, the virus is replicating but symptoms haven’t appeared yet. Viral shedding, the period when an infected person is actively releasing virus, peaks between one day before symptoms start and about 5 days after onset. After that, the ability to pass the virus to others drops steeply between days 5 and 9 of illness.
This timing matters because people are most contagious right around when symptoms first appear. The basic reproduction number (R0) for the original 2003 SARS outbreak was estimated between 2 and 5, meaning each infected person spread the virus to roughly 2 to 5 others before control measures kicked in.
Superspreading Events
SARS transmission is not evenly distributed. A pattern sometimes called the “80/20 rule” holds that roughly 20% of infected people drive more than 80% of all transmission. These superspreading events tend to occur when a highly infectious individual spends extended time in a crowded, enclosed environment with poor ventilation. During the 2003 outbreak, a handful of superspreaders were responsible for chains of infection that reached dozens or even hundreds of people in hospitals, hotels, and apartment buildings.
Survival on Surfaces
SARS coronaviruses can persist on surfaces, though how long varies widely depending on the material and temperature. At room temperature (around 22°C), the virus survives longest on smooth, non-porous surfaces.
- Plastic: Half-life of 4 to 10 hours at room temperature, extending to several days in cold conditions (5°C or below).
- Stainless steel: Half-life of roughly 3 hours, with survival up to 14 days in some studies.
- Glass: Half-life of about 4 hours, survival around 4 days at room temperature.
- Cardboard: Half-life of 5 to 12 hours.
- Fabric and clothing: Half-life ranges from under 1 hour on nylon to 2 days on cotton.
- Copper: Half-life of less than 1 hour, making it one of the least hospitable surfaces.
Porous materials like paper and fabric tend to inactivate the virus faster than hard, smooth surfaces. Cold temperatures extend survival significantly across all materials. While surface contact (touching a contaminated object and then your face) is a possible route, it’s considered a secondary pathway compared to direct respiratory exposure.
The Amoy Gardens Outbreak and Fecal Aerosol Spread
One of the most striking examples of an unusual transmission route occurred at the Amoy Gardens housing complex in Hong Kong during the 2003 outbreak. More than 300 residents became infected, and investigators traced the spread to fecal aerosols traveling through the building’s plumbing system. When an infected resident flushed a toilet, virus-laden bioaerosols were generated inside the vertical drainage stack due to hydraulic pressure. These aerosols then escaped through floor drains with dried-out water seals and traveled via exhaust air and vent pipes to other apartments in the high-rise.
This fecal-respiratory route was unusual but demonstrated that SARS coronaviruses present in stool can become airborne under the right plumbing conditions. Similar concerns resurfaced during the COVID-19 pandemic, when a comparable pattern was identified in another high-rise building.
Hospital Transmission and Protective Equipment
Hospitals were major amplification points during the 2003 SARS outbreak, partly because certain medical procedures generate large quantities of aerosols. Mechanical ventilation with a breathing tube is one of the strongest clinical predictors of airborne virus in a patient’s room. Procedures like suctioning the airway or collecting respiratory specimens can trigger coughing that releases an aerosol plume, and positive-pressure ventilation can actively disperse viral particles into the surrounding air.
Protective equipment makes a dramatic difference when used correctly. Studies found that healthcare workers using inadequate or reused protective gear had infection rates as high as 49% to 73%, while proper equipment significantly reduced risk. Failures in protection, combined with poor ventilation in older hospital wards, fueled some of the largest nosocomial clusters during the 2003 epidemic.
Factors That Increase or Decrease Spread
Several environmental and behavioral factors influence how easily SARS transmits. Indoor settings with recirculated air and limited fresh airflow create the highest risk. Cold, dry conditions help the virus survive longer both in the air and on surfaces. Crowding and prolonged close contact, especially during activities that produce heavy breathing or vocalization, compound the danger.
Conversely, good ventilation (open windows, high air exchange rates, or filtration systems) dilutes airborne viral particles quickly. Warmer temperatures and higher humidity reduce surface survival. Regular handwashing addresses the surface-contact route, and properly fitted respiratory protection cuts exposure to both droplets and aerosols. The combination of these measures is what ultimately brought the 2003 SARS outbreak under control within months of its emergence.