The EPA says that we normally spend 90% of our time indoors. With the climate cooling down, the time indoors increases. This is a significant problem, as the air indoors is more polluted than the air outdoors.
Though an air-filtration system is an ideal step, it is not the only step you can take to maintain a green environment. A simple and cost-efficient step is to reduce and/or remove pollution sources. Though complete elimination is not possible (a reason you need an air-filtration system), reducing emissions helps.
Since the air outdoors is healthier than indoor air, a good ventilation system also helps. However, the air outdoors still isn’t that clean.
An air-purifier is an ideal solution, but it never hurts to take the extra steps.