Sir, use of thermal screening has resurfaced recently in the light of the COVID-19 pandemic. It has been widely accepted worldwide for mass screening to restrict patients with high grade fever entering specific areas during high infectious periods. The concept of thermal imaging is not new and dates back to 1963 when Barnes et al.1 first demonstrated its usefulness in diagnosing physical illness. Sherman et al.2 concluded that infrared thermography (IRT) is the simplest method of detecting body temperature in which thermal radiation emitted by a surface is captured by an infrared camera and converted to temperature.

Oral, axillary and inguinal temperatures are perhaps the most common sites apart from rectal and oesophageal. However, all these sites normally underestimate true core body temperature. They also require the measuring device to have direct contact with the subject which is undesirable in mass screening in a pandemic era for fear of spreading infection. The development of non-contact infrared thermal images of the head to determine body core temperature is clearly very promising. Recent data indicated that the temperature of the inner canthus of the eye is consistently the warmest area on the head and the most suitable site for use in fever detection, however this is complicated as it is also affected by climatic conditions. There will be always be incubation situations when infectious persons may not have an elevated body core temperature for several days and so will not be picked up by a screening system. Likewise there will be persons who have regained normal body temperature following a bout of fever yet who may still be infectious or who have suppressed their elevated body temperatures with pharmaceutical agents such as aspirin or paracetamol.

Despite this some authorities, for example the USA, maintain that fever screening may reduce spread statistics by up to 50%. The incorrect use and failure to follow guidelines for devices used are also causes for concern. Several administrative institutes and hospitals have recently used thermal infrared imaging techniques for mass screening but at high risk of bias, hence the need for large placebo-controlled clinical trials before any technique can be recommended.