In depth: From carbon dioxide monitors to ultraviolet light: Ways to improve ventilation to prevent Covid-19 and TB
We look into the role of ventilation in the transmission of airborne diseases such as tuberculosis and Covid-19 and assess what can be done to improve ventilation in buildings.
Ventilation plays an important role in preventing the transmission of airborne disease but, currently, ventilation standards for buildings are focused on preventing odours in spaces, instead of preventing disease transmission. Both tuberculosis (TB) and Covid-19 can infect an individual via airborne transmission.
A recent article published in the journal Science stressed the need for a drastic shift in how we view the role of ventilation in the transmission of disease globally. It states that the ventilation standards, guidelines and regulations that buildings should meet should specify “minimum ventilation rates and other measures to provide an acceptable indoor air quality (IAQ) for most occupants”. These requirements focus on controlling odour and “occupant-generated bioeffluents” such as carbon dioxide (CO2), as well as providing thermal comfort.
“For decades, the focus of architects and building engineers was on thermal comfort, odour control, perceived air quality, initial investment cost, energy use and other performance issues, whereas infection control was neglected,” the authors write.
The article also states that the only buildings where airborne infection control happens are in healthcare facilities, as ventilation rates are generally higher than in other public buildings.
Ventilation standards for buildings in SA
According to Peta de Jager, a professional architect and impact area manager at the Council for Scientific and Industrial Research (CSIR), ventilation standards in South Africa do not prevent infection by airborne diseases as the standards were set to ensure that there is enough air to breathe in a room and to control odour.
“South Africa has a comprehensive set of building regulations and standards applicable to all classes of spaces,” she says. “These standards stipulate the number of litres (of air) per second, per person needed to ventilate spaces.”
De Jager says that ventilation standards for buildings are determined by the National Building Regulations and Building Standards Act of 1977. These are elaborated as the South African National Standards (SANS) 10400, which provides a comprehensive guide to complying with the regulations.
Natural vs mechanical ventilation
De Jager explains that there are two common ways of achieving ventilation rates, and the success and effectiveness of a ventilation system come down to the design, engineering, maintenance and operation of the system.
Natural ventilation can be achieved through windows or any other openings or apertures in a building that allow air to enter a space naturally. Another way is through mechanical ventilation, which is when a building is a closed envelope and some form of fan or duct or combination is used to introduce or force new air into a space while taking the “old” air out.
She says that either of these, or a combination of both, are used to ventilate a building.
The role of ventilation in transmission of airborne diseases
Prof Robin Wood, Emeritus Professor of Medicine at the University of Cape Town, who heads a research team involved with aerobiology (the transmission of diseases between people through the air), says there should be an emphasis on disease transmission when buildings are designed and built.
“I think it’s been forgotten about for many years. We’ve been very interested in decreasing building costs and not the functional requirements for stopping disease transmission and I think probably that’s what Covid-19 may have taught us,” he says.
Explaining airborne transmission, Wood says it takes place through a pathogen that lives in an infected person moving through the air to infect someone else.
But the conditions need to be right for there to be transmission.
Wood says the organism needs to survive in the space between the two people and needs to arrive at that individual in the right package and go to the right part of the respiratory tract to cause disease.
Both TB and Covid-19 can infect an individual via airborne transmission, he says, but there are differences between the two pathogens that make TB particles much better at long-term airborne survival than Covid-19 (SARS-CoV-2) particles.
A factor that can determine effective transmission of one of these pathogens is the number of infected particles in the air and the dilution factor, says Wood.
He explains that dilution of the airborne particles, that comes from when an infected person breathes out, occurs when those particles mix with fresh or uncontaminated air. Dilution already happens automatically as the infected person breathes out because the particles move away from that person and become less concentrated as the distance increases.
When someone exhales, according to Wood, they send out a warm plume of moist air, which travels as a unit.
“Within that moist plume are little particles and the particles change their size as they dry up, as the humidity dries,” he says.
The particles that survive are the ones that can infect someone via airborne transmission.
“The ones we’re talking about are the ones less than five micrometres in diameter. They are the ones that distribute through a room and therefore ventilation affects that,” he says.
The amount of dilution of those particles is what affects the risk of transmission, according to Wood. Enough dilution, which is affected by the amount of ventilation in a space, can reduce the probability of transmission.
Wood explains that the risk of transmission is increased if there are many people in a small space, as the chances of these small particles being transmitted are increased. One or two people in a room with good ventilation decreases the risk of transmission. But the risk is never zero.
“That’s really what we’re talking about with ventilation. How much air are we breathing from other people and how much fresh, pathogen-free air are we getting into the space?”
Two ways of measuring
Wood outlines two ways that ventilation can be measured. The first is air changes per hour, which looks at how many times air gets exchanged in a room.
“That gives you a rough idea of the ventilation quantity of that room, but it’s irrelevant if there are a lot of people there or the room is small,” he says.
From a disease transmission perspective, the second method is more useful. This is what Wood refers to as the per-person ventilation.
“What you’re really interested in there, is the per-person ventilation… how much ventilation does each person get and that can be measured in carbon dioxide, for instance,” he says.
Practical ways to improve ventilation
To improve ventilation to reduce the risk of disease transmission, the article in Science recommends starting by acknowledging the problem.
“First and foremost, the continuous global hazard of airborne respiratory infection must be recognised so the risk can be controlled,” the article states. It further recommends that guidelines should be changed, both in the Global World Health Organization IAQ guidelines and in national guidelines or standards in each country.
Using CO2 monitors to improve ventilation
Here in South Africa, Wood proposes some practical measures. One way of potentially improving ventilation, he says, is the use of carbon dioxide monitors in buildings.
CO2 monitors, according to Wood, are a useful way of measuring the per-person ventilation in a space, as CO2 acts in much the same way as the infected small particles do, and is easy to measure.
“It (CO2) is the measure of the output of each individual in the room and (the) eventual per person ventilation, and I think that’s probably more directly related to transmission risk than, for instance, air changes per hour,” he says.
Putting such a monitor in a building will help determine whether the ventilation in the building is sufficient.
“It is an easy way of determining that ventilation is satisfactory and you can have them with alarms. It would be a relatively cheap technology and can pretty accurately give you an indication of the per-person ventilation and the absolute risk (of disease transmission),” he says.
Both Wood and De Jager propose a multi-faceted approach to improving ventilation in buildings.
De Jager distinguishes between the design of a building and how it is operated. She says a ventilation issue is not always the result of a flaw in the design, but instead in how the building is used.
“The best, most effective infection control measures are baskets of measures… they work in synergy. If you add them all together, they are more effective than each one is individually,” she says.
De Jager suggests conducting a critical assessment of existing buildings to determine whether it complies with the ventilation standards outlined in the National Buildings Regulation Act and if not, then changes need to be made.
She says it is important to look at the occupancy levels of the buildings and take steps to reduce those levels. This can be done using systems like the ones that are implemented during the Covid-19 pandemic. De Jager suggests appointment systems, as well as any system that will help reduce the time people are in contact with each other.
De Jager also recommends that facility managers of buildings conduct a systematic risk assessment of the building. She then proposes looking at a hierarchy of controls by assessing current measures like administrative and engineering controls, and determining whether more controls should be added. She then suggests looking at whether personal protective equipment might be necessary.
Structural changes to improve ventilation
For Wood’s multi-pronged approach, he says “the structural things we need to do are complex and they vary from adding ultraviolet lights, putting in mechanical ventilation systems… but ideally, natural ventilation systems are the best way and in the long term the cheapest way.”
According to him, ultraviolet lights can be a good alternative to ventilation if structural changes cannot be made in a building because they can kill pathogens such as viruses or TB.
“One complication is UV lights require a lot of maintenance. They need to be checked regularly, but it is an alternative if the building itself can’t increase the ventilation rates,” he says.
He also adds that natural ventilation, particularly windows, can be problematic when the weather is cold as people tend to close them. But it could be countered by a CO2 monitor, because it will inform people that the levels are too high, and that ventilation needs to be improved.
Wood remarks that building standards are a good starting point, but they don’t necessarily provide for enough per-person ventilation within a building or room.
One issue he raises is that not all windows provide the same amount of ventilation, but all windows are represented in the same way on a building plan. He says windows that allow cross-flow across a room are more efficient than windows on a single side of a room. He adds that high windows are much more efficient for ventilation than low windows, due to upward thermal air movement. DM/MC
This article was produced by Spotlight – health journalism in the public interest.
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