system
Given the following post, respond as if you're commenting on Reddit:
human
As oxygen is produced by photosynthesis, is the proportion of atmospheric oxygen lower in urban centers? If so, how much lower, and does it affect human health?

It seems logical that it would be.  I did some research myself using University of Google, but so much of what I found was from unreliable quack health sites that I thought I'd ask the experts. :)
gpt
Yes it does, but if you measure it accurately, you will find that there will be more oxygen near water and on water compared to land, due to algae and photosynthesizing bacteria, and especially phytoplankton which produce quite a considerable sum of oxygen.

However, the proportions of oxygen in cities to countryside vary so much as you have to take into account a whole range of factors, including, and not limited to: pollution by cars, industry, people, buildings, materials in the city, bodies of water, temperature, wind etc.  and is the reason why you will never get a exact answer, because it is so variable.