Thermometer Guns are Notoriously Inaccurate. Why Then Do We Use Them?

Weary faces and crumpled shirts spotted with coloured dots: these are sights that are common on an evening M.R.T. ride these days. There has been talk of a new normal in Singapore. The virus is here to stay, but life has to go on, albeit with more caution. Synonymous with this new normal are the temperature screening stations popping up at malls, workplaces and government facilities.

Small pistol-like infrared thermometer ‘guns’ have been deployed before in efforts to stop SARS and West African Ebola containment measures. The only difference this time around is the scale of deployment. Across the world, countries are using thermometer guns at train stations, airports, hospitals and even grocery stores, in an attempt to identify potential carriers of the virus. Singapore is no exception. 

For Mr. Ng, a security guard tasked with temperature taking, the measures seem ineffective. “The temperatures go up and down very randomly. Many have got temperatures of 34-35 degrees and there are little fevers picked up.”  To put that into context, at body temperatures below 35 degrees, you are likely to have hypothermia — a condition which eventually leads to the complete failure of your heart and respiratory system and even death. While some might attribute Mr. Ng’s readings to faulty equipment, there seems to be a wider trend here. An air-conditioner contractor also remarked that his temperature varies drastically throughout the day, sometimes in short successions. “These thermometers no use la, troublesome only.” 

He might have a point. Experts like Dr. Jamer Lawler, a medical expert at the University of Nebraska’s Global Center for Health Security, argue that these thermometers are “notoriously not accurate and reliable.” Why is this the case? Why do we still use them if they are inaccurate? These are the questions we answer in today’s explained.

How Do Thermometer Guns Work?

To answer that question, we first need to have a proper understanding of how these thermometer guns work. 

You may recall from your secondary school science lessons that anything with a temperature above absolute zero (−273.15 °C)  has molecules inside of it moving around. The higher the temperature of the material, the faster these molecules move. As they move and collide into each other, infrared radiation (IR) is emitted. This radiation is exactly what thermometer guns measure. A lens inside its barrel focuses the infrared light emitted from an object onto a detector called the thermopile. The thermopile then absorbs the infrared radiation and turns into electricity. The larger the amount of infrared light, the higher the amount of electricity produced. On the final step, electricity output is measured and displayed on the screen.

Why Can They Be Inaccurate?

Medical staff check each other’s temperatures at Italy’s border with Slovenia. (Source)

There are three reasons as to why these guns might be ineffective in catching COVID-19 carriers.

Firstly, the technical limitations of IR thermometers could lead to imprecise readings. Many a time, thermometer gun wielders hold them too far from the subject’s forehead, generating unusually low temperature readings. Other times, they hold it too close, leading to fever-level readings. While each thermometer’s sweetspot varies, generally, these guns work best at a distance of about 3-5cm. People tasked with temperature taking either might not know about this or have too many people’s readings to take. This is exacerbated by the fact that these readings could be affected by frost, moisture, dust, fog, smoke, other particles in the air and changes in ambient temperature. A 2008 experiment conducted at a large Paris hospital proves that these variables drastically affect results. When 2026 patients had both their both forehead temperature and eardrum temperature (which is used as a reference or ‘gold standard’) taken, correlation between the two was low. The IR thermometer underestimated body temperature at low values and overestimated it at high values. When plotted on a graph, the lack of correlation is pretty dramatic:

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Thermometer gun on the vertical axis and the eardrum thermometer on the horizontal axis. (Source)

Medication might also help potential COVID-19 carriers suppress their symptoms. Paracetamols like Panadol help reduce body temperature and the chance of being detected. Some, who might want to avoid travel restrictions and social distancing measures, might deliberately take these medications. This has already happened in China, where in Qinghai Province police are investigating one man on suspicion that he passed through checkpoints by covering up his ailments. Others might consume these drugs unmaliciously, thinking they just have a passing flu or fever. 

The virus itself poses a few issues too. According to The New England Journal of Medicine, SARS-CoV-2 carriers might take days to develop a fever. In these initial days, thermal screening is of no use. Singapore’s SAFRA Jurong Cluster clearly illustrates this point. While everyone who attended the Feb 15 private function at Joy Garden restaurant had their temperatures checked, none were detected to have a fever. This together with the fact that there seems to be patients who are asymptomatic (have no flu, fever etc) but are still shedding the virus, makes thermal detection even harder.

Why Then Do We Still Use Them?

A temperature screening station in the CBD. (Source)

In the security and defence industry, there is something called a security theatre. It’s not a physical space or an actual performance — but a concept. Put simply, it’s the act of enacting countermeasures intended to provide the feeling of improved security while doing little or nothing to achieve it. Many have criticised bag searches on mass transit systems – like Singapore’s M.R.T. – as being part of a security theater as there is a low probability of terrorists getting caught in the first place. There are just too little searches being done for it to be truly effective.

There are a few benefits to these theatres though. The presence of security guards checking bags on the M.R.T. assures passengers that they are safe and that action is being taken by the government. During times of high anxiety and risk, it allows people to carry on with activities (like taking the M.R.T.) they would have otherwise avoided. On a second level, the show of force might also deter potential terrorists (especially already nervy ones) from pulling the trigger. 

While the virus isn’t sentient or choosy over who it attacks, thermometers guns might form part of a pandemic preparedness theater. Government efforts at contact tracing and surveillance are largely invisible to the public. These activities might constitute some of the most important steps in a pandemic, but are still underappreciated. By instituting thermal screening stations, the government and private stakeholders are showing that tangible action is being taken. The presence of these stations also act as a signaling mechanism. Since they are rather intrusive – some people get screened multiple times in a day – it tells the public that life cannot proceed as per normal. We are dealing with a real crisis and these are daily reminders of that. 

The stations might also be a test of last resort. When talking about the effectiveness of temperature screening after all members of the SAFRA Jurong cluster passed them, Health Minister Gan Kim Yong argued that “We need to look at the various measures as a whole package and not any single particular measure.” Beyond the travel declaration forms and other social distancing measures that are already meant to filter potential virus carriers, the temperature screening might constitute one final layer of protection. Considering that they are relatively inexpensive to set up – most firms already have IR thermometers in their inventories – it might be worthwhile to continue engaging in these measures, even if it results in catching of just one or two COVID-19 shedders, because that’s all that’s needed to start another cluster. Studies have also shown that while thermal screening is ineffective in picking up diseases like Ebola and SARS, it has been effective in picking up dengue fever in Taiwan for example. While no COVID-19 carrier has been identified worldwide by these guns, it is just too early to tell whether they might be of help. 

Even an entire country might need a Placebo pill sometimes.

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