Arcing voltage thru air varies a lot since air varies a lot. However, about 1 kV/mm is a rough guide. Of course you want to be well below the arcing voltage on a circuit board. At half that, 220 µm would mean 110 V, so 36 V sounds safe enough.
Usually the PCB pads extend a little past the device pins, so the clearance between pads is the limiting factor.
A comment by W5VO reminded me of something I should have mentioned.
What I said above is a very rough guide that you want to derate heavily. That may be good enough for personal projects. However, for real commercial products, there are likely regulatory requirements that apply. Some of these apply by law, while others are standards you have to meet in order to get third part approvals. Most of the time there is little practical distinction. They are simply rules you have to follow.
There are different requirements depending on what the device is intended for, where it will be used, and what third party certification the buyer or a reseller insists on. For example, patient-touching medical devices in use in the United States must meet certain creapage and clearance distances by law. These tend to be much more conservative that other rules. Intrinsic Safety (standard for electrical equipment in hazardous locations) also uses much more conservative rules than for something like office equipment.
W5VO mentioned IPC rules, but there are also rules from the IEC, local electrical codes, private certifying agencies like UL and FM, etc. Sometimes the hardest part of the design is figuring out what standards actually apply.