The purpose of this short note is to provide some background on infrared temperature measurement and infrared imaging, from longstanding technical reports and personal experience (nonhuman applications), as the industry considers using these tools as part of a comprehensive SARS-CoV-2 protective response and preventive control in the produce supply chain.
Non-contact temperature screening of on-farm labor, food packing and processing employees, and essential contractors and visitors, including auditors and inspectors, has been suggested in multiple forums as a recommended screening for individuals infected with SARS-CoV-2 (Covid 19). Infrared Temperature (IRT) measurement and Infrared Digital Temperature Imaging (IDTI), or IR Thermography, have the potential to provide some level of limited invasive screening of individuals or ambulatory scanning of groups of people. While it is reported from numerous clinical cases, at this point, that mild to high fever is experienced in the majority of infected and symptomatic persons, the range of temperature above population normal [NIH reports 36.1°C (97°F) to 37.2°C (99°F)] to meet the current standard for a clinical fever at 38 °C (100.4 °F). Without an individual’s baseline normal measured temperature, it is difficult to determine whether an Elevated Body Temperature (EBT) scan indicates a fever and one of several indicating symptoms of SARS-CoV-2 infection. In addition, in the data-fog of this pandemic, it is difficult to be confident of the information overload on the percent of infected but asymptomatic individuals who are readily capable of transmission to others, especially if social/physical distancing is poorly or not practiced. Conservatively, because a lot of the available data cited by experts is disclaimed as known to be impacted by both false positives and negatives early in the rolling progression, lets agree on 10%.
The key factor is that IRT and, especially, IDTI technologies may be a helpful ancillary tool capable of detecting an individual showing indications of EBT. This is particularly attractive in cases where individual privacy and confidentiality are primary and speed and low cost to deploy are needed. Group scanning has been used in public areas, such as airports, train and bus terminals, and other gatherings with variable success. It is very important to acknowledge and accept that any non-contact thermal measurement, while potentially informational if detecting a presumptive EBT, is imperfect. Only a trained and licensed medical response practitioner or health professional can and must make a confirmatory determination if a person is ill and specifically infected with SARS-CoV-2.
Infrared Temperature and Emissivity (ε)
The values reported digitally, or accompanied by a graphic “heat map”, reflect the surface emissivity, or its thermal emission property. For brevity, this is not the place to go into a deep explanation of thermal radiation from a surface and how knowing the emissivity characteristics of that surface is essential for establishing an accurate reading. Regarding human skin emissivity, it is known to be high ( = 0.95 to 0.98) in comparison to an entirely black, non-reflective object’s surface with a value of 1. Therefore, for quick screening, no elaborate correction factor is essential and the “rule of thumb” is human surface temperature +2°C. Detailed medical and medical engineering studies have reported that the emissivity of human skin is negligibly impacted by skin color. There are, of course, sources of personal and near-environmental variability. For medical applications, complex studies provide algorithms and models and corrections but for the less precise applications being discussed here, these are not practical to include.
Key studies have reported that a fixed distance and standardized facial target area are best. Recommendations, even for general commercial applications, indicate that the area around the forehead and inclusive of the eyes with a pinpoint IRT at near-contact distance for individualized screening (Fig 1.) and fixed-point high resolution, high pixel density with IDTI for individual or group scanning. IRDT imaging needs to be close to perpendicular to the target area for greatest accuracy. High pixel density is optimal to capture the temperature profile close to the tear duct areas, in expanded images, which has been correlated with body core temperature in several studies. Those interested in adopting this screening or scanning approach as one tool in a comprehensive prevention program should consult a technical expert with the many knowledgeable industrial service support providers and public health agencies or professionals familiar with this widely used thermal measurement technology.
For now, until PMA learns of a broad interest among the industry to evaluate the application of, especially IDTI (Fig 2.), please follow this link to a short PDF of application examples taken from postharvest produce safety applied research in the Suslow Lab at UC Davis.
The above image was taken with this mid-cost range handheld FLIR device