Most paint removal or building cleaning is either done with harsh solvents or harsh blasted granules like sand. The solvents are not good for the environment, and the sand can remove important outer layers on materials like brick.
We visited a cleaning/stripping company in Toronto, Ontario called ProBlast that gets the job done in an efficient but gentler way. (By the way, Pro-Blast is not the same company and I don't recommend them.) When they need chemicals, they use organic solvents. Most amazing is that when they need to 'sand blast' they don't use sand. They use corn cob granules. Unlike silica sand that can actually etch into brick and stone, the corn cob granules are sharp enough to clean off the surface but not gritty enough to damage material below. They use this technique for blasting paint off of cars or cleaning paint and other grime off of homes and commercial buildings. They are able to do a through but gentle job on both brick and wood siding.
Besides being ecologically responsible, they have a most beneficial marketing technique -- their local franchises across the country will clean up local churches and community centres for free -- from cleaning walls to removing graffiti.