Breathing is part and parcel of the human condition. If you don't breathe, you can't really do much. The air around us, however, is not exactly as pure as it once was. Arguably, the air around us was never really that pure in the first place, but that's delving a little too deep. In past ages, people sometimes feared drinking water during times of war because there was a very good chance it was poisoned by the enemy. An underhanded tactic, to be sure, but one that was used nonetheless. In the modern world, people may not be as paranoid about the air they breathe, but there is no denying that things like second-hand smoke and the possibility of airborne bacterial infection make breathing a hazardous thing.
You've probably heard a horror story or two about the potential side effects of breathing in second-hand smoke. You've probably been told that, since there's no filter to reduce the amount of toxins and carcinogens, you're at greater risk than smokers if you allow yourself to be exposed regularly. Smokers have the filters on their cigars and cigarettes to reduce the damage, but people who stand there and take in the second-hand smoke don't have that protection. The potential consequences of prolonged exposure consists of a list of respiratory disorders covering everything from minor fits of coughing to something as severe as lung cancer.
Of course, it isn't just the smoke from cigarettes and the like that you have to worry about. The modern atmosphere has, according to some estimates, more carbon dioxide and carbon monoxide than before. Some skeptics doubt the veracity of the claim that the air now is more toxic than at any other point in history, but that is mainly because there is a lack of evidence to prove for or against the theory. While radical, some people see the air quality of the typical urban community as being hazardous to a person's health in the long-term, primarily due to the toxic components in it.
Science also has an impressive list of all sorts of airborne pathogens, bacteria, and viruses that are floating around. While most of them are relatively minor and carry only a minimal risk of bacterial infection, others are not as benign in nature. However, it doesn't take a deliberate terrorist attack for someone to contract an illness by just breathing in the pathogens. Things like the common cold can be contracted because the viral or bacterial infection is airborne. While the common cold is something that everyone has to put up with and is far from lethal, it can do enough damage to the body's immune system that other bacteria can step in and cause more damage. For some cases, such as Legionnaire's Disease, all it really needs is an opening to do the dirty work.
According to some statistics, the amount of allergens in the air seems to be showing an increase over the past few decades. When allergen levels from 1968 were compared to levels in 1996 and 2006, there was a notable increasing trend in between. This problem can sometimes be more aggravated in well-developed urban environments, particularly in large-scale office buildings with internal climate control systems. Most office buildings also have centralized cooling systems that regulate the temperature of the entire building. The exhaust and intake ports of these systems can be perfect breeding grounds not only for various types of bacteria, but also a variety of allergens. As anyone who has an allergy understands, the intensity of the reactions can vary, but most of them inevitably cause discomfort and minor health concerns.