The use of carbon dioxide sensors inside cars is a good way of controlling the ventilation but it's also possible to detect people inside a parked vehicle.
A traditional way of using a CO₂ sensor is to know when to force air from the outside to the inside of the car. If the car is traveling in the forest the outside air quality is most likely better in terms of pollution compared to the air in on a busy street in the town. If the car monitor the outside air quality it is possible to weigh the inside air quality to the outside air quality. If only one person is traveling in the car there is not as much CO₂ exhaled into the interior.
Another way of using the same sensor technology in a closed space, such as a car interior, is to monitor the level of CO₂ and see if it is increasing even though the car should be empty. If even the slightest increase of CO₂ is detected it is clear that the car is not empty. A dog or baby left in a car during a hot day (or even a cool day) is in danger of being exposed to deadly temperatures. Little does the ambient temperature have to do with the rise of inside temperature, it only effect the starting temperature. A general rule of thumb is that the temperature inside a car (even with crank windows) rise 1℃ per 1 minute. Within 10 minutes the inside car temperature reach deadly levels (close to or above body temperature), children and dogs have a hard time coping with that level of body temperature and can easily faint making it even harder for bypassing people to detect their urgent situation. A CO₂ based alarm system could force down windows, sound the alarm or force the environment control system to activate.
To see what levels of CO₂ a child or animal contribute when inside a car Senseair has made a test using a family of two adults, two children (8 month and 4 years old) and a dog. The car used was a Volvo V70. As reference a driver plus one, two, or three passengers were also monitored in the same car. Ambient air temperature was around 0℃. The results are shown in the graph below.
As seen in the graph the alone child, infant or dog are the lowest in carbon dioxide increase. But they will lift the CO₂ levels with about 200 - 400 ppm during a 10-minute period. To further understand that this small but significant change in CO₂ levels we can plot the levels of an unoccupied car and see what happens to the CO₂.
The four people inside the car started with circulation off during the first 8 minutes. When the circulation was back on, the ppm levels dropped almost 1000 ppm. This can be explained by pockets of fresh air was mixed with the higher CO₂ air. After another 14 minutes all people exited the car and a quick drop by 2000 ppm is seen. With an empty car the CO₂ molecules exits the car with an average flow of 8 ppm per minute. With this in mind it is easy to see that even an small increase in CO₂ is asignon of a child or animal left inside a car.
To explain the last part of the graph, after 51 minutes the car door is opened and a technician enters the vehicle and start the rigging of the next test.
The car was equipped with 18 gas sensors and it is found that the best response time is in the center of the car, between the two front seats (seen as a red dot in the image above).