The “Pig Pen” Effect: When Motion, Not Breathing, Spreads Infection

Back in the 1950s, cartoonist Charles Schulz introduced a new character into his world-famous Peanuts strip: Pig-Pen, an amiable boy known for the cloud of dirt and dust that follows him everywhere.

“I’m a dust magnet!” Pig-Pen declared.

In truth, we’re all dust magnets, and this fact may play a role in the airborne spread of disease.

Research shows airborne pathogens, having landed on surfaces, can be kicked back up into the air by human activity — walking a hallway, opening a door, removing a lab coat. These dangerous microbes, riding on dust particles, can then be inhaled or swallowed.

Influenza, norovirus, and Clostridium difficile are likely transmissible in this manner, and SARS-CoV-2 may be as well. Pig-Pen himself has been invoked in scientific discussion on transmission of the flu and Covid-19.

“Aerosols can settle to the ground, where they are resuspended into air just by us walking around,” Linsey Marr, Ph.D., an American environmental engineer, explained at an international workshop on Covid transmission. “[This causes] a secondary opportunity for transmission by aerosols. We call this the Pig-Pen effect.”

We know that sneezing, coughing, singing, talking, and mere breathing can transmit Covid-19. It’s assumed that SARS-CoV-2 also can be transmitted via contaminated surfaces, known as fomites. Now, evidence suggests another possible route: “aerosolized fomites” — in other words, the Pig-Pen effect.

“We showed that just rubbing a [flu-]contaminated paper tissue aerosolizes several thousand micron-scale particles,” William Ristenpart, PhD, a chemical engineer at the University of California, reported at the international workshop. “What this points to is, there are other ways a virus can get into the air.”

Ristenpart’s studies on flu spread among guinea pigs points in the same direction, as does coronavirus research conducted at Wuhan hospitals.

Ristenpart calls aerosolized fomite transmission “underappreciated” and notes that his team’s findings, published in Nature, have important implications for public health, including the spread of Covid-19.

This is especially true for hospitals, nursing homes, businesses — any high-traffic where air is shared. Certainly, masks can reduce the number of particles exhaled by infected people, and enhanced surface cleaning can help contain spread via fomite. However, controlling disease transmission via the Pig-Pen effect requires additional precautions, including continual air dis-infection.

Walking Kicks up Millions of Particles Per Minute        

In a slide shown at the international Covid-19 workshop, three women, one of them infectious, stand in a room filled with virus-laden dust particles. The particles, some of them resuspended by footsteps, are represented as red dots, to help us imagine what we can’t see.

But those scattered red dots don’t adequately represent the swarm of aerosols typically kicked up by human activity.

“An adult walking across the floor can resuspend 10 to 100 million particles per minute, while tossing and turning on a mattress can stir-up similar levels of microbial-laden dust,” explains Brandon Boor, PhD, an aerosol scientist at Purdue University in the United States.

In a study conducted with Finnish microbiologists, Boor found that in 1 minute of crawling across a carpet, 1,000 to 10,000 resuspended particles will deposit in the infant respiratory system.

Boor is another scientist who references Charles Schulz in his work, noting the cartoonist’s portrayal of Pig-Pen as being perpetually enveloped by a dust cloud is “quite accurate.” Boor calls resuspension of particles via human movement a “major indoor source of biological particulate matter,” including bacteria, fungi, and allergens.

Well before the emergence of SARS-CoV-2, research suggested infectious aerosols could be bandied about by human activity.

In a British hospital study, for example, researchers sampled the air near patients infected with Clostridium difficile. The bacteria, known to cause nausea and severe diarrhoea, has become a deadly threat in hospitals due to the emergence of virulent, antibiotic-resistant strains.

What the study found: Over a 10-hour period, airborne C. difficile was most commonly detected during periods of activity, such as food delivery, ward rounds, or bedding changes.

“Activities known to liberate particles into the air, such as bed making and curtain drawing” may contribute to the spread and aerosolization of C. difficile, the scientists concluded.

Unlike SARS-CoV-2 or influenza, C. difficile is not a respiratory pathogen; people become infected by swallowing airborne particles or touching contaminated surfaces and then touching their mouths. Given that patients’ clothing, bedding, and skin are often teeming with C. difficile spores, aerosolization of surface particles could be quite dangerous.

Norovirus, another virulent and highly contagious stomach bug, can be resuspended off the floor, as well.

A Brazilian research team, reporting in the Journal of Hospital Infection, evaluated protocols for cleaning floors contaminated with norovirus. The scientists detected Norovirus particles in 27 of 36 post-cleaning air samples, concluding the virus “can be aerosolized during floor cleaning, and its particles may be inhaled and then swallowed or can settle on surfaces.”

Can SARS-CoV-2 Spread via the Pig-Pen Effect?

It’s clear that pathogens can be dislodged from surfaces and resuspended into the air. But can these “liberated” aerosols actually cause infection?

Apparently so, according to a clever University of California study on the spread of influenza among guinea pigs.

The study involved 12 pairs of guinea pigs, housed in separate but adjoining cages. One rodent was immune to influenza, having previously been infected; its partner remained vulnerable to the disease. Using a paintbrush, researchers applied liquified influenza to each immune rodent’s fur, ears, and paws.

The cage design prevented contact between the rodents while directing air to flow from the immune rodent’s cage to its partner’s cage. This left just one plausible infection source for the vulnerable rodents: viral particles launched airborne by the activity, rather than exhalation, of their immune partners.

The results surprised even the scientists: After one week, 3 of the 12 vulnerable guinea pigs had contracted influenza.

Based on the study, co-author William Ristenpart disputes the assumption that infectious airborne flu particles must come from breathing, coughing, or sneezing or from aerosol-generating medical procedures.

“We now provide direct experimental evidence that . . . aerosolized fomites from a virus-contaminated environment can spread influenza viruses through the air,” his team wrote.

In the same paper, Ristenpart reported that viable influenza particles can be aerosolized from paper tissue. His team found that crumpling, folding, and rubbing dried, contaminated tissue released up to 900 particles per second and that the particles were “in the respirable range.”

If influenza can be spread via the Pig-Pen effect, can SARS-CoV-2?

Ristenpart’s team suspects so. Their guinea-pig study references research conducted at two Wuhan hospitals dedicated to Covid-19 care.

In one study, researchers captured air samples in several locations throughout the hospitals. They found the highest concentration of coronavirus RNA not in Covid wards or patient isolation rooms but rather in rooms where medical staff changed out of their gowns, masks, and other gear.

The suspected source: “resuspension of virus-laden aerosols from the surface of the protective apparel worn by medical staff while they are removing the equipment.”

The virus-containing aerosols were small enough to remain suspended in the air for long periods and to be inhaled.

Continual Air Dis-infection: Critical for Limiting Infection Spread

As Brandon Boor notes, the resuspension of dust is “an inherently transient process,” changing minute by minute based on human occupancy and movement patterns.

What this means for hospitals, nursing homes, retail, or office spaces: air dis-infection units must be running 24/7.

Surface cleaning is important, but pathogens rebound quickly. One study found hospital bed rails would have to be professionally cleaned every 2 hours to keep harmful bacteria to a safe level.

And while masks effectively limit emissions of dangerous particles, public mask-wearing regulations are often flouted, and the rules will disappear, at any rate, when a Covid-19 vaccine becomes widely available. SARS-CoV-2, however, will continue to float about, along with influenza, norovirus, and countless other pathogens that contaminate our shared air.

In the indoor environment, controlling infection will always require multiple strategies. Critical among these is 24/7 air dis-infection, particularly NanoStrike technology from Novaerus.

Novaerus units generate an electrical discharge, known as ultra-low energy plasma, that destroys viral, bacterial, and fungal particles. Within nanoseconds, the devices obliterate pathogens into inert, harmless debris, expelling clean air back into the room.

Unique among air-disinfection devices, NanoStrike technology leaves behind no by-products and is safe to operate continually even in ICUs and operating theatres.

Independent lab tests have confirmed the technology destroys MS2 Bacteriophage, a surrogate for SARS-CoV-2, as well as influenza, Clostridium difficile, and numerous other airborne pathogens. The same technology also wipes out the pollutants, such as volatile organic compounds (VOCs), that cling to dust particles.

In an early Peanuts strip, Pig-Pen is described as the only person who could manage to get dirty while walking in a snowstorm. He simply can’t shake off the dust.

In reality, none of us can, which makes Novaerus technology essential for hospitals, schools, pharmacies, and pubs alike.