31 July 2017 by Phillip Johnston • Last updated 14 December 2021
One of the aspects I enjoy about embedded systems is that many products will be physically handled by your target audience. Knowing that a person is going to use your product informs many aspects of design: your software must be stable, your interfaces must be intuitive, and your hardware must be safe to handle.
Modern devices are being increasingly reduced in size. As size is reduced, thermal constraints become much more important and much harder to dissipate. However, burning people does not bode well for our product, so it’s important to set temperature limits where people will touch the product.
Many system designers set their product’s skin temperature limits based on “onset of damage” to a person. Often this standard is used to gain as much computational performance as possible: stricter thermal constraints require us to scale back on power.
However, I think that my products should not cause users any potential damage. After some digging, I found out that NASA agrees. NASA has created a new approach for defining touch temperature limits which can be used to estimate safe touch temperatures from one second to infinite durations. NASA now advocates “onset of pain” as the standard they currently use for spacecraft design.
The authors found many strange details with the previous touch temperature standards. For instance, NASA’s previous lower temperature limit did not trace to any studies or data, and their upper temperature limits used a single value for all materials. They sought to improve NASA’s touch temperature standards and back the recommendations with data. The paper describes an approach for setting temperature limits based on contact time (e.g. 1-10s or infinite), which can be used for determining temperature limits for your own products.
For commonly used materials, 45°C appears to be a safe hot touch temperature limit:
The left hand termini of the lines in the figure correspond to the properties of aluminum. Since none of the lines dip below 45°C (113°F), a material temperature limit of 45°C (113°F) can be used as a first screening point for all commonly used materials. That is, if the temperature of any commonly used material is lower than 45°C (113°F), it will meet the hot touch temperature requirements for all contact times.
For cold temperature limits, 10°C is the recommendation. Lower temperatures quickly lead to onset of pain, numbness, and frostbite.
The tests showed that a hand skin temperature of 10°C (50°F) was tolerable [12], so this was taken as the skin temperature limit.
Our products should not cause users harm. Reconsider your team’s touch temperature standards on future products. Let’s improve the standard for the devices we plan to put in people’s hands.
I’ve included my highlights from the paper below, but you can also see my annotated copy of the paper if you’re interested.
Starting with a summary of the problem:
Defining touch temperature limits for skin contact with both hot and cold objects is important to prevent pain and skin damage, which may affect task performance or become a safety concern. Pain and skin damage depend on the skin temperature during contact, which depends on the contact thermal conductance, the object’s initial temperature, and its material properties. However, previous spacecraft standards have incorrectly defined touch temperature limits in terms of a single object temperature value for all materials, or have provided limited material-specific values which do not cover the gamut of likely designs. A new approach has been developed for updated NASA standards, which defines touch temperature limits in terms of skin temperature at pain onset for bare skin contact with hot and cold objects. The authors have developed an analytical verification method for safe hot and cold object temperatures for contact times from 1 second to infinity.
Touch temperature standards in an important detail to consider in our system design:
Correctly defining touch temperature limits for spacecraft equipment protects the crew from pain and physical harm while optimizing crew and system performance. If the touch temperature limits are set too conservatively, the use of operational constraints, such as wearing gloves, may be mandated unnecessarily— increasing the time required to perform tasks. Alternately, thermal coverings may be mandated for powered equipment that would impede their heat transfer to the cabin — decreasing their reliability by causing them to run hotter than is truly required. If the touch temperature limits are set too liberally, the crew may experience pain or skin damage while handling equipment. Therefore, it is important that spacecraft touch temperature limits be defined correctly.
NASA’s original hot touch temperature limits were based on human testing with heated aluminum plates. The data showed that pain onset occurred at an aluminum plate temperature of 45°C (113°F).
This temperature was established as the limit for all materials for any contact time from zero to infinity and was the standard until 1995, when two changes were made:
- Material—specific hot temperature limits were established for times between 1 and 10 seconds.
- A cold temperature limit of -18°C (0°F) for unlimited contact with all materials was added.
Scary that the previous lower temperature limit does not track to any real data:
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. An updated hot and cold touch temperature standard, backed by data, was needed.
“Onset of damage” is a common way to define temperature limits. However, NASA argues this is not ideal:
When defining touch temperature limits, the question arises as to whether pain or skin damage should be the limiting factor. The danger of allowing skin damage is that it may affect a person’s ability to use the affected area, including hands or fingers, which may lead to decreased performance in critical areas such as controlling a vehicle.
NASA agrees that damage is not a good way to define temperature limits:
NASA holds to a higher standard, using the onset of pain as the crew protection limit.
The epidermal/dermal interface was chosen as the critical location because only the dermis is sensitive to pain. The epidermis is insensitive to temperature owing to its lack of pain receptors.
Given that the data on pain converge around the same value, the 44°C (111.2°F) epidermal/dermal interface temperature derived by Hatton and Halfdanarson from Stoll et al.’s data should be used as the upper limit for contact with hot objects. A subtle, but important distinction when defining touch temperature limits is that they should be based on the epidermal/dermal interface temperature at pain onset. Some previous standards have incorrectly defined the allowable object temperature as the used the skin temperature limit, which is overly conservative.
45°C is a safe temperature for common materials:
The left hand termini of the lines in the figure correspond to the properties of aluminum. Since none of the lines dip below 45°C (113°F), a material temperature limit of 45°C (113°F) can be used as a first screening point for all commonly used materials. That is, if the temperature of any commonly used material is lower than 45°C (113°F), it will meet the hot touch temperature requirements for all contact times.
The original NASA cold temperature limit is too low, and damage can actually occur before the cold temperature limit is reached:
The origin of NASA’s cold temperature limit of -18°C (0°F) for any material for unlimited contact time is unclear, and upon reviewing the literature it was found that this limit is likely to cause both pain and skin damage. Research on human tolerance to cold has shown that onset of pain occurs at 15°C (59°F) skin temperature (Havenith et al. [10]), numbness occurs at 7°C (44.6°F) (Provins and Morton [11]) and risk of frostbite is risked at 0°C (327) (Havenith et al. [10]).
10°C is the new recommended lower touch temperature limit
The tests showed that a hand skin temperature of 10°C (50°F) was tolerable [12], so this was taken as the skin temperature limit.
The conclusion provides insight into why we care so much about defining temperature standards for the human hand:
These limits pertain primarily to intentional contact with the hands and fingers, as some parts of the body may be slightly more sensitive to pain owing to their thinner epidermis. However, the hands and fingers are the most likely parts of the body to be unprotected and used for grasping and manipulation of objects.