While many of the toxic substances we have expelled into our environment since the Industrial Revolution can be found anywhere, including in polar bears in the Arctic Circle, there is no denying that the air quality in city areas is poorer than that of the suburbs. Anyone who has visited Tokyo in Japan will realise the extremes it can reach. Air filters are commonly worn over the nose and mouth there, and oxygen can be purchased from dispensers in public areas. Although I believe it is improving now with stringent vehicle emissions laws.
Poor air quality is believed to be one of the primary causes of increasing asthma rates, particularly in children. Just one of the reasons many couples ready to start their families seriously consider moving to the suburbs to do so. Chronic respiratory complaints like asthma are often compounded if the sufferer is also overweight, but that may actually be more likely to occur in the suburbs.
City life generally functions at a faster pace. Even those city dwellers with no concern over their fitness levels tend to walk further and use stairs more often than their suburban counterparts. In the suburbs the places we want to go tend to be further away, hopping in the car and driving there is more convenient and less time consuming.
Those concerned about their health and fitness, however, can usually find the means to exercise to their hearts content no matter where they live. But those in the suburbs who jog, often make the mistake of doing so alongside busy roads, hammering car exhaust fumes into their lungs. Most in the city are forced to use parks, whether they want to or not, because the footpaths are too congested with pedestrians.
Central city areas have a much higher population density than the surrounding suburbs; so when it comes to infectious diseases, the risk of catching one is significantly greater than in the suburbs. Whether from direct contact with a sufferer or simply passing through where they have been before us. Too few people stay home when they have an infectious respiratory disease like a cold or influenza. City dwellers are far more at risk from epidemics than suburbanites.
Violent crime can happen anywhere, particularly home invasions, but muggings tend to be much more common in cities. Car accidents on the other hand, are more likely to impact those of us who live in the suburbs; because we use cars more often and can usually drive faster than it is possible to do in city streets.
Essentially, both cities and suburbs have their pros and cons when it comes to our health. It's six of one and a half dozen of the other, in effect. All we can really do is make ourselves aware of potential hazards particular to our own situations and take suitable procautions to try and avoid them.