In the late 70's before unleaded gasoline and emissions testing. From that vantage point every building had the first two floors looking like the picture and as you went up they got cleaner and cleaner. From then on I understood why so many city dwellers love black clothes, not because they are chic but to save on dry cleaning.

flying in the air, luckily... I guess?

I hope they have protective gear. Maybe they need to put it on!

The threat it represents is all the people that were breathing all that in before it settled down.

That said, I'd still wear one, and it ought to be a requirement they be provided them by whoever they work for with an option to wear them if they want.

Most buildings in the U.S. don't look like this much anymore - especially in NYC. I could see if this photo was dated in the 70s, when they really started cleaning off the soot from the 1800s through to the 1950s when many urban buildings were covered in it. But I'm skeptical this is solely caused by coal plants, fireplaces and cars - especially if it was allowed to stay like that for decades.

So, my guess: there was a fire.

Like when it was taken and where.

Coal hasn't been used to generate power in or near New York City for decades (since the early 70's). Diesel fuel quality and diesel engines themselves have improved considerably since the Clean Air Act was passed (1970), as have gasoline quality and car engines. NYC's air quality has gotten much better in the last 40 years.

Most of my more than 40 years of working have been in Manhattan and I have no recollection of ever seeing a building being power washed. Sidewalks yes, but not buildings. The building in the picture was probably built in the late 19th century and may never have been washed before. If that's true, it's going to be dirty, but the dirt accumulated over more than a 100 year timeframe.

A last point is 9/11. When the WTC came down, it created an enormous dust cloud that cover all of lower Manhattan. At least some of the dirt was likely deposited at that time.

The building does look great after being washed - it would be nice if that became a trend and more building got a facelift.

I didn't think it was coal dust. I have seen buildings power washed.

It was started by Moose in London:

and is popular here too:

This is an example of what is revealed when cleaned Fifth Ave has some of the best

lots of built-up grit on them that makes them look dark.

For example - this church in downtown Cleveland looked like this for many years:


After a thorough cleaning in recent years, it looks like this:

We have been watching this on our local PBS channel this year off and on. They have been busy scrubbing and rehabbing the village more each year and it is very evident when they started again this fall at episode one.

Where the ceiling was not cleaned when they refurbished it.

The rest of the station was cleaned and made quite beautiful in the 1990s.