Window washing actually helps improve air quality in your apartment or house in New York City.

Do you like opening your windows to let some fresh air in? Well if, your windows aren’t clean, the air you are letting in might not be all that fresh. The city grime, that we are all used to see everywhere in New York, has been found to be a significant source of toxic indoor air contamination! Turns out, according to atmospheric chemist James Donaldson and colleagues at the University of Toronto, the urban surface films, when exposed to sunlight, release nitrogen oxides and active forms of nitrogen compounds into the air. These compounds are carried straight into your home by the drafts.

Much of the toxic nitrogen-derivatives byproducts of fossil fuels combustion by the automobiles and power plants after some naturally occurring reactions end up trapped in the city grime or urban surface films in inert forms (not directly dangerous). When the Sun hits the grime, the nitrogen compounds dissolve forming toxic gases like nitrogen dioxide. This reaction is called "renoxification" and mostly takes place on the surfaces of the buildings where exposure of the grime to the sun is the biggest.

This research, among other things, is supposed to help improve modeling the air quality in cities, by factoring in the surface films accumulated on the buildings. Though it it easy enough to model what the grime+sun reaction happening on our windows does to the air inside our homes.

Another thing to point out is that the grime on the windows contributes to glass corrosion , as one of its components is nitric acid.

source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1964914/pdf/ehp0114-a00444.pdf