Obviously you can't do such a simplistic direct comparison between horse emissions and car emissions. How does leaded petrol play into this comparison, particulates, etc.
You're totally right, my comment was far too simplistic. However it captures the feelings at the time, cars at that time were far "cleaner" in that they smelled a lot better, did not clog streets with excrement etc
Obviously play the car out a few decades and the emissions were _not_ so clean.
