Does the air in your house sometimes smell like smoke? Perhaps your neighbors heat their houses by burning wood, or perhaps you live close to a smoke stack, a power plant or a factory. Air pollution is one of the leading causes of deaths today, so if you can smell the smoke, there's a good chance you're inhaling toxic levels of microscopic particles in your lungs. If your house has a ventilation unit (an air exchange system), I have a solution for you. A solution which has drastically decreased the problem for our household.

Step 1: Design

The standard way of preventing smoke from coming in is using some very costly filtering equipment, in the range of hundreds or thousands of dollars. Since a normal household doesn't require the airflow to be constant, a smarter solution is to simply stop the ventilation when the incoming air is bad, and resume it when the air is better. This system costs about 50$ in parts.

The design of my system is the following:

Detect the quality of the air coming in. I'm using a Plantower pms5003 sensor that detects the small particles coming from the smoke. Because my house's ventilation unit has both the incoming air and the outgoing air ducts, I've installed this small sensor (about the size of a match box) inside the incoming air duct, just after the air is heated. If your house gets its air from vents in the outside walls or windows, you'll need to install it somewhere right* outside your house, the closer to your air vent(s), the better. The Plantower sensor that I used for this project can detect particles bigger than 0.3um, which is enough to detect at least wood smoke.
Detect the air quality inside. Use the same type of sensor, installed in a central location inside your house, open to detect the air you're breathing, definitely not inside a closed cupboard.
Wire the ventilation fans to a relay (an electrically operated switch).
Stop the airflow if incoming air quality is bad. For this task I have an arduino mega board (a tiny programmable computer) to which I've connected both the incoming and indoor sensors, and the fan relays. If the air coming into the house is much worse than the air inside, the program calls for the relays to stop the ventilation fans. Notice that this system can only stop periodic bad air, such as a gust of wind blowing smoke to your house from a factory, or a puff of smoke from a neighbor's chimney.

Step 2: Additional Fine Points for the Programming

The arduino code (savu.ino) I've coded for our own project is included to help you get started, but you're going to need to program your own depending on your needs and electronics.
The program should not stop the ventilation for too long a time though, otherwise the air quality will suffer from low oxygen and high carbon dioxide levels, and the building may suffer damage from a buildup of humidity. To avoid this problem, I made my system stop for 15 minutes at maximum, and after that if the outside air quality is still bad, it will make the fans run at lowered speeds.
Since the sensors can get dirty by accumulating dust. To be able to correct for this dust pile up, I've added an adjustable value to offset the sensors from each other. Probably especially the outside sensor will at some point start to think the air is permanently smoky due to dust buildup. It can either be cleaned after some years with compressed air, or replaced.
Sometimes air outside can be much worse than the inside air for days at a time, in my location it can be caused by smoke buildup in the area due to weather, or smoke particles coming in from far away countries even. Since the system will improve indoor air quality by shutting off the worst of the outside air, it has a tendency to stop the fans permanently during such days of bad air. To combat this, I'm also measuring an average of the incoming air quality during the last 24 hours, and using that to figure if the bad air coming in right now is actually a sudden short event that we can do something about, or if it's just the normal air quality today.

savu.ino
Download

Step 3: Results

Previously our house used to smell like a chimney during cold winter days, due to the neighbors using their fireplaces. After installing this system we can no longer smell the smoke inside. Measurements from the particle sensor in the device show that before I used to see about 5000 to 10000 size 0.3µm (or larger) particles per 0.1 liter of air inside the house. After this project the numbers have dropped considerably, down to 500 - 1000 if the outside air is particularly heavy, and much less on clearer days.

Now I'm going to take a break from combating gusts of smoke, and get back to programming my role-playing game Driftmoon. I hope this project will help not only us, but you as well, to enjoy fresher and healthier air inside your house. :)