The Waterfront already is seriously affected by air pollution from the Gardiner Expressway and city traffic, and the Island Airport adds significantly to that pollution.
A 2004 Toronto study by the Board of Health found there were 1,700 premature deaths and 6,000 hospitalizations every year as a result of air pollution in the city. In 2014 this had fallen to 1,300 deaths and 3,550 hospitalizations.
Air pollution still kills four times the number of people in Toronto than homicides and traffic deaths combined and yet it rarely receives a mention in the media.
It has been well established that air pollution from airports can lead to a number of health related problems from cancers to repertory illnesses. Children are particularly vulnerable.
The Toronto Board of Health voted unanimously against the Porter jet proposal based on a report of the health impact. You can find that report here.
Useful links of studies relating health issues to airports
National Bureau of Economic Research (U.S.) report on Airports, Air Pollution, and Contemporaneous Health
International Civil Aviation Authority Organization Airport Air Quality Manual
Airports, Air Pollution, and Contemporaneous Health (U.S.) by Wolfram Schlenker and W. Reed Walker