If you have concerns you should get tested - state lab generally does this sort of thing for very low cost (around here it was about $20) Is this the interior wall of an apt? Could be new plaster skim coated over old.... but I have heard of asbestos sometimes being added to plaster so its hard to say.

It's probably the grey base coat and the "scratch coat".

When they plastered over the brick, they applied a thick coat of grey plaster to fill in the mortar lines and smooth it out. Then they applied a thinner grey coat and literally scratched it to make it rough enough to hold the final coat - the white (more expensive) layer that most people think of as "plaster".

interior wall. this is behind the boiler so I only noticed it recently and I think it's not good for it to be in the open like that. if it's been painted recently and the outside plaster is in good condition, is it a concern elsewhere in the apartment?

If asbestos is sealed in by other material and is unlikely to be disturbed, it is usually not a significant concern. For example asbestos floor tile can usually be encapsulated by sealing the tile and adding another floor covering (wood, sheet vinyl, etc.) on top.

Bruce

If it's not deteriorating (like getting crumbly and falling to pieces), and you're not physically messing around with it, then it is not a concern, even if it contains asbestos. I'm careful about environmental hazards, but this does not look like a problem to me. And, although only testing would reveal if it had asbestos, I am doubtful that it does. What is that material attached to (i.e., is that a brick wall or a chimney, or what}? You also say it is well-hidden, so why is it an issue if no one is likely to get access to it or likely be messing with it?

If asbestos is present, it is a problem when you start doing things like dry-scraping popcorn ceilings, or sanding down vinyl asbestos tile, or taking a crowbar to furnace insulation, etc. Not when it is sitting quietly undisturbed.