“We sometimes freeze the specimen with liquid nitrogen, which is extremely cold, you know. This is another technique we use now – but the specimens are not alive.” — Lennart Nilsson
I recently facilitated a HazOp where the process required both very hot heat transfer media (steam and hot oil) and very cold heat transfer media (low temperature brine and liquid nitrogen). Everyone knew that the hot pipes were too hot to touch and workers needed to be protected, typically with insulation for personal protection. We used 60 C (140°F) below an elevation of 2.13 meters (7 feet) as the criteria for requiring insulation. (Yes, everyone understood that these are not explicit OSHA requirements, but they were convenient criteria that didn’t have to be explained.)
However, we didn’t know about cold surfaces. How cold must a surface be to require insulation for personal protection? Pretty cold, it seemed. After all, don’t bartenders serve ice-cold beer throughout their shift without injury?
Federal Standards
When it comes to safety in the U.S. workplace, OSHA is the go-to federal agency. While OSHA doesn’t have any regulations on the subject, OSHA has guidelines for working in cold environments and will enforce compliance using the general duty clause. Despite having requirements to protect from cold stress and hypothermia, OSHA doesn’t have any standards, recommendations, policies, or letters of interpretation about protection from contact with cold surfaces.
Does anyone?
The federal agency that has invested the most effort in understanding working in extreme environments is NASA. That includes contact with cold surfaces. Early in NASA’s history, the agency determined that the cold temperature limit for surface contact was -18 C (0°F). Later, two NASA researchers set out to learn where that limit originated. They determined that
NASA’s cold touch temperature limit of -18°C (0°F) does not trace to any studies or data, and appears to go beyond anecdotal limits for pain. Anyone who has tried to select their desired drink from the bottom of an ice-filled cooler has observed that pain results after much less than a minute of immersion in 0°C (32°F) ice water.
So, there was an early federal standard for how cold surfaces could be and skin contact be allowed. One that doesn’t stand up to scrutiny.
Europeans Weigh In
The question remained. In 2006, Geng and colleagues, published work they did at the request of The European Commission, in their article “Temperature Limit Values for Touching Cold Surfaces with the Fingertip”. They were primarily concerned with three harmful skin temperatures and the surface temperatures that could result in them. The three harmful skin temperatures were
- 15 C (59°F) – Results in the onset of pain
- 7 C (44.6°F) – Results in the onset of numbness
- 0 C (32°F) – Results in frostbite
They acknowledged that these skin temperatures did not result from momentary contact with surfaces at those temperatures. Because of the roles that contact time and the thermal inertia of the contact surface play, the temperature of the surface could be lower before resulting in the injury.
The few seconds it takes to serve an ice-cold bottle of beer does not result in frostbite.
Thermal Inertia
Thermal inertia, also known as thermal effusivity and as thermal responsivity, is a measure of a material’s resistance to temperature change. A material that is resistant to temperature change will also be resistant to changing the temperature of something that comes in contact with it. Thermal inertia is equal to the square root of the product of thermal conductivity, density, and specific heat capacity:
r = (k · ρ · c)0.5
Geng’s work used aluminum, stainless steel, nylon-6, and mahogany. Here is the thermal inertia of these materials, along with soda-lime glass (since beer bottles are also a valuable point of reference):
Because of differences in thermal inertia, different materials will have different impacts on skin temperature and so the likelihood of injury. The metals, with their high thermal inertia, can result in frostbite in 2 to 6 seconds when they are at -15 C (5°F). Nylon-6 and mahogany, on the other hand, can be as cold as -35 C (-31°F) and require 15 to 65 seconds before the onset of numbness.
Geng’s work became the basis of ISO 13732-3:2005, Ergonomics of the thermal environment-Methods for the assessment of human responses to contact with surfaces – Part 3: Cold.
The New NASA Cold Temperature Limit Value
NASA did not leave -18 C (0°F) as its cold temperature limit value. After reviewing Geng’s work and other research, NASA now advises 10 C (50°F) as its cold temperature limit value for surfaces in which skin will be in contact. As the discussion on thermal inertia indicates, however, it is possible for skin to contact colder surfaces safely, depending on how long the contact lasts and the thermal inertia of the surface.
About Serving Beer
It should be clear from Geng’s work that it is safer to serve beer in glass bottles than in aluminum cans, at least from the perspective of surface temperature. Moreover, the proper temperature for serving beer is 3.3 C to 12.8 C (38°F to 55°F), colder for pilsners and warmer for beers with more complex flavor profiles, like stouts. This puts beer at a safe temperature, regardless of the signs outside taverns that say, “coldest beer in town.”
