Because of the risk of TB infection through airborne transmission, one way to decrease the chances of this is by improving the ventilation or airflow in an enclosed space, and diluting the air and TB molecules in the process.
But, writes Elri Voigt for Spotlight, how do we know when exactly ventilation in a room falls to dangerously low levels?
One solution, suggest local scientists, is to use portable carbon dioxide (CO2) monitors.
A recent study set out to test whether there is a correlation between the CO2 levels to which a healthcare worker is exposed, and their exposure to TB.
The employees – who all worked in settings with a high risk of TB exposure – were asked to wear personal CO2 monitors for the research, which was conducted at Tygerberg Hospital, Cape Town, by the Clinical Mycobacteriology and Epidemiology (CLIME) Group, a division of the Molecular Biology and Human Genetics Department at the University of Stellenbosch.
The findings were published last year in the journal Clinical Infectious Diseases.
They confirmed an association between a high CO2 reading and exposure to TB among the hospital staff.
High TB exposure
Professor Grant Theron, a professor of molecular biology and human genetics at the Faculty of Medicine and Health Sciences at Stellenbosch University and primary investigator for the study, said medical staff face a huge risk of acquiring TB because facilities are often filled with people with undiagnosed TB and thus untreated TB.
These individuals then spread live TB molecules when they exhale, filling the air with TB.
The study, said Theron, looked at 138 staff in Tygerberg Hospital between August 2017 and June 2019. The sample consisted of mostly nurses and some doctors, with the majority (34%) having less than five years’ experience in hospital settings.
All participants underwent an interferon-gamma release assay (IGRA) test when the study started. In simple terms, this is where a blood sample is taken and exposed to dead TB in a petri dish and, based on how the cells in the blood samples react, it can be determined whether that person had been exposed to TB before or not.
The participants were retested a year later. (It is important to note that a positive IGRA test does not mean someone necessarily has TB disease, only that they have been exposed to TB before).
When first enrolled, 74 of the 138 participants had never been exposed to TB: 37 of those 74 were then chosen to carry personal CO2 monitors with them for a year. A year later, when retested, 25 of the 74 who had never been exposed to TB now tested IGRA positive.
Theron said this means 34% of those individuals who had never been exposed to TB had encountered it in that timeframe.
“Which means that since joining Tygerberg Hospital, 34% had had their body exposed to live TB, and that’s an extremely high rate,” he said.
“Now, that in itself is not a new finding. We know that health workers traditionally carry a huge occupational risk for TB.”
Looking for a correlation
This is where the CO2 monitors become important.
Professor Robin Wood, emeritus professor of medicine at the University of Cape Town, said the monitors measured how much CO2 was in the air in a room. This finding would be related to the size of that room, number of people in it, how much CO2 they are breathing out, and the ventilation present.
“So essentially, it’s a good marker of per person ventilation. It’s telling you how much fresh air is being supplied for each individual, but you can also use it to estimate the amount of air people are breathing from other people…
“If they’re in close proximity that can give you an idea of how much air exchange there is between people, and certainly respiratory diseases are usually dependent on the crowding and the amount of air that you’re breathing from other people, particularly if one of them is infectious,” he said.
CO2 readings can be used to model the risk of infection with a respiratory disease, but it is only indirectly showing the per person ventilation.
“The main source of carbon dioxide in buildings is air exhaled by other people, and the more carbon dioxide in the air, the more exhaled breath. And presumably, if there’s more exhaled breath, there is more TB in the air, assuming you have people with undiagnosed TB in facilities,” Theron said.
Essentially, a low CO2 reading means there is good ventilation in a room, and if TB molecules are present, these are diluted and thus less likely to be inhaled by another person.
The researchers found that among the staff who had gone from an IGRA negative to an IGRA positive test, the CO2 readings on their personal monitors were more likely to be higher than those who had remained IGRA negative.
This, said Theron, confirmed the hypothesis that the more re-breathed air or CO2 someone was exposed to, the higher the odds of them being exposed to TB.
“So that is an association. It’s not a causal relationship, but I think it’s very logical… it suggests that interventions reducing personal CO2 exposure and improving ventilation can reduce infection in health workers,” he said.
Is it feasible in the real world?
When the study was conducted, said Theron, personal CO2 monitors weren’t readily available off the shelf, but since the pandemic, they have become more available. They cost about R1 000-R2 000 for a consumer-grade version.
It is thus feasible to consider equipping healthcare workers with personal CO2 monitors, he added.
One reason it should be a very acceptable and affordable expense is that the cost of having a health worker out of action in a country like South Africa – due to TB – is so damaging to the system, and to that person,” he pointed out.
The study results have several implications, he added. The first is that ventilation in healthcare facilities needs to be improved. He also suggested mask-wearing for both staff and patients.
“Because we know in South Africa that even if you are coming to the hospital or clinic for your HIV treatment, there’s a good chance you have undiagnosed TB. And a good precaution, a good way of preventing TB from spreading, in addition to ventilation, is just wearing a mask.”
The second implication is that personal CO2 monitors can be a tool used to empower healthcare workers and give them the ability to point out areas that are dangerous for them.
Giving them personal CO2 monitors would act in the same way as a radiation dosimeter used to measure an employee’s exposure to radiation.
“The thinking was that it allows staff to be advocates for their own safety and their own working conditions. If you have something that clips on your belt, which allows you to self-identify where risky areas might be, where there is poor ventilation, not only does that enable you to help avoid those areas, but it also allows you to advocate for change within your local environment,” Theron said.
“By improving their working environments, these employees are also reducing transmission to other patients.”
Study details
Predicting airborne infection risk: association between personal ambient carbon dioxide level monitoring and incidence of tuberculosis infection in SA health workers
Ruvandhi Nathavitharana, Hridesh Mishra, Amanda Sullivan, Shelley Hurwitz, Philip Lederer, Jack Meintjes, Edward Nardell, and Grant Theron.
Published in Clinical Infectious Diseases on 26 March 2022
Abstract
Background
High rates of tuberculosis (TB) transmission occur in hospitals in high-incidence countries, yet there is no validated way to evaluate the impact of hospital design and function on airborne infection risk. We hypothesised that personal ambient carbon dioxide (CO2) monitoring could serve as a surrogate measure of rebreathed air exposure associated with TB infection risk in health workers (HWs).
Methods
We analysed baseline and repeat (12-month) interferon-γ release assay (IGRA) results in 138 HWs in Cape Town, South Africa. A random subset of HWs with a baseline negative QuantiFERON Plus (QFT-Plus) underwent personal ambient CO2 monitoring.
Results
Annual incidence of TB infection (IGRA conversion) was high (34%). Junior doctors were less likely to have a positive baseline IGRA than other HWs (OR, 0.26; P = .005) but had similar IGRA conversion risk. IGRA converters experienced higher median CO2 levels compared to IGRA non-converters using quantitative QFT-Plus thresholds of ≥0.35 IU/mL (P < .02) or ≥1 IU/mL (P < .01). Median CO2 levels were predictive of IGRA conversion (odds ratio [OR], 2.04; P = .04, ≥1 IU/mL threshold). Ordinal logistic regression demonstrated that the odds of a higher repeat quantitative IGRA result increased by almost 2-fold (OR, 1.81; P = .01) per 100 ppm unit increase in median CO2 levels, suggesting a dose-dependent response.
Conclusions
HWs face high occupational TB risk. Increasing median CO2 levels (indicative of poor ventilation and/or high occupancy) were associated with higher likelihood of HW TB infection. Personal ambient CO2 monitoring may help target interventions to decrease TB transmission in healthcare facilities and help HWs self-monitor occupational risk, with implications for other airborne infections including coronavirus disease 2019.
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