# Were any toxic metals used in the International Space Station?

Have any of the following metals been used in the ISS?

• Lead: used in some batteries, and in many types of solder.
• Mercury: used in some batteries, and in tilt switches (which I would expect have no use on the ISS).
• Cadmium: used in some batteries, and in some photoresistors.
• Arsenic: used in some LEDs.

If there is a policy prohibiting one or more of them, a citation would be useful.

• I can't find any direct mentions of the use of any of those materials but there is this document about "Space Station Requirements for Materials and Processes" which covers the topic. On page 28 there are mentions of Cadmium and Mercury. Lead itself is heavy so it would be avoided as much as possible anyway and apart of that it isn't really toxic in it's solid form. – GittingGud May 13 '19 at 12:33
• Berylium is another toxic metal used in space ships, for instance the Mercury capsule. – Uwe May 13 '19 at 14:01
• Lead solder is still considered the best solution, reliability wise, for aerospace grade electronics. Also, in some types of machinery you will need weights, and since you will need the weights to be compact on that kind of vehicle, lead (or even DU!!) would be a likely candidate.... – rackandboneman May 13 '19 at 15:28
• this and this suggest that Cadmium is unsuitable for vacuum applications due to off-gassing, and Lead is unsuitable for high temperature vacuum applications for the same reason. Tin-Silver solder is suggested to be a better choice for vacuum. – SamYonnou May 13 '19 at 15:34
• @GittingGud But tin whiskers might be avoided even more. – chrylis -cautiouslyoptimistic- May 13 '19 at 17:00

The Urine Processor in the Regenerative Environmental Control and Life Support System uses hexavalent chromium as a pretreatment solution for the urine.

• +1 As your toxin (Cr(VI) (aq)) is a heck of a lot nastier than my toxin (As as GaAs crystal) I think you should get more votes. – uhoh May 13 '19 at 23:46

note: protests in comments have led me to add the following sentence. Arsenic is a toxic metal and arsenic is used on the ISS, but the arsenic used on the ISS as discussed below is not a toxic metal per se, instead it's a nasty, extremely toxic semiconductor.

GaAs is soluble in HCl which means if you eat it you are eating soluble arsenic. So don't eat it.

Also see this comment.

GaAs and AlGaAs are the standard substrate and heterostructure materials for infrared, red and yellow LEDs and lasers. Indicator lights and optical proximity and interrupt sensors are probably quite numerous and will all have arsenic.

You can see the red LED component in the ISS' new RGB LED lighting in @OrganicMarble's answer to Why are these astronauts green?

UHF and microwave transmitters and receiver front ends for everything from communications and data to radar ranging for spacecraft docking to GPS are likely to have some high-speed GaAs bipolar transistor devices in them.

Arsenic has been a common dopant for a half-century in silicon based electronics and even the substrate wafers themselves can sometimes be moderately doped.

• @Dr Sheldon: Not only is it embedded, the amounts used as dopants are miniscule and the actual LED is on the order of a micrometer thick. Remember that the dose makes the poison: there just isn't enough in the LEDs to be harmful. – jamesqf May 13 '19 at 16:33
• @jamesqf in the case of GaAs (red LEDs, bipolar RF chips) it's the substrate itself that is gallium arsenide, the arsenic is not just a dopant, it's half the wafer! While GaN (blue and white light LEDs) tends to be a thin epitaxial layer on sapphire, the GaAs parts are made from the wafers themselves. – uhoh May 13 '19 at 16:35
• @CBredlow While many satellites use higher efficiency dual and triple junction PV cells that include III-V materials, the ISS' panels are just silicon. See answers to Why does the ISS not use the most efficient solar panels available? as well as How are the silicon PV cells constructed in the ISS's solar panels? Are they as flexible as they appear here? I don't know if there is arsenic doping in them. – uhoh May 13 '19 at 17:50
• The LED wafers used in typical indicator LEDs are pretty small, too -- I don't have an exact size, but it's on the same scale as a grain of salt. Easily under 1% of the volume of the LED. – duskwuff -inactive- May 14 '19 at 1:07
• @uhoh: Gallium arsenide is not arsenic, any more than carbon dioxide is oxygen. To get into the body, you'd somehow have to decompose it into gallium & arsenic. – jamesqf May 14 '19 at 5:18

The bearings on the CMG (Control Moment Gyro) rotors use beryllium, which is pretty toxic when you machine it.

• Also, heatsinks and ceramic parts in high performance radio and radar gear tend to use beryllium oxide, which is even more toxic if you disintegrate this (brittle!!) material in any way.... Also, bearings are somewhat self machining.... – rackandboneman May 13 '19 at 15:24
• @rackandboneman Not even high performance: your microwave oven magnetron has BeO2 ceramic insulators. You recognize it by pink coloring. It's safe unless you're grinding it. – user71659 May 13 '19 at 22:10
• A compact 800W transmitter is high performance RF stuff, even if you use it to heat a non-aerospace grade soup. – rackandboneman May 14 '19 at 8:19

The heating system has ammonia in one of its loops, which is very irritating to human eyes, nose, throat, and lungs. On Earth, you can just leave spilled ammonia for a few hours, and it will waft away into the atmosphere until it's at a low enough concentration to be tolerable. In a space habitat, it would be terrible. You might be able to get everyone into space suits, evacuate the atmosphere, and refill the atmosphere. However, the ammonia would seep into fabric and plastic and be difficult to completely exhaust.

Ammonia is a good refrigerant, and it's still usable below the freezing temperature of water, which is why they tolerate it on the ISS. The 2019 news about neopentyl glycol being a viable solid refrigerant is great... if it's usable as a refrigerant at both cold and human-range temperatures, then spacecraft cooling can be made safer and more reliable.

• Ammonia isn't a metal under normal conditions, but it certainly can be noxious. – uhoh May 13 '19 at 23:48