This is possibly one of the most common things that homeowners forget about. And yea I know that not everyone is looking up at their ceiling or in their furnace to see if their filter is clean. But it has to be a habit or a necessity. Filters not only filter the air that you’re breathing but can also be a problem to your airflow. Initially when filters were made, they were made to protect the unit not the person(s) living in the home or building. They were made to protect the evaporator coil from debris and from plugging it up because of again...airflow. The restriction of airflow is very important. Let’s take for example, during the summer when you’re using your air conditioner. If the filter is plugged up it will literally start icing up the evaporator coil which in turn restricts the airflow and in turn doesn’t cool down the home anymore. And in the winter when using your heater or furnace and the filter is plugged up, yep guess again, airflow problems. But this time the furnace or heater will start shutting off due to high limit switches because it’ll start overheating. There’s not enough airflow to pass through your heat exchanger and it could also possibly cause your heat exchanger overtime, to crack or fracture. Which is dangerous and will cause carbon monoxide(c0), to seep through your home by following the air stream. So lack of airflow during the summer is more of an energy problem and the lack of airflow in the winter is a safety problem. So there you go. This subject could be expanded to other areas but for now I believe you get what I mean.