Fireworks cause significant, short-term pollution. Large firework displays cause more significant amounts of pollution for a longer period of time. The reason why they are not considered for ban by the EPA is that their health risks/exposure are for a short period of time over the course of a year. Of course, banning fireworks would go over like a cabbage fart in an elevator.
Some of the exposures include unburned hydrocarbons (smoky smell), metals (such as chrome, lead, barium, copper, iron, nickel, cobalt), perchlorate, acid chlorides, nitrogen-oxides (NOx) and acids, sulfur-oxides (SOx) and acids, particulate, and carbon monoxide. And of course, since many are made in China, who knows what other stuff is in there from time to time.
For sensitive individuals, viewing should be done at a distance and infants/young children should not be seated underneath or downwind from the festivities.
The last link below is actual air quality results during a fireworks display.
I hope this answers your question.