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Do you catch a bus and finish the trip with a head ache or nausea? Many people put it down to the stress or motion sickness, but it may well be the recirculated air is slowly poisoning you.

The easiest way to control air temperature is to stop outside air from getting in, especially on very hot or cold days. The only problem is that this also traps the existing air in and when you have a large number of people occupying that space, the air quality can quickly be compromised.

But how can you measure air quality? When the biggest influence on air quality are people, measuring carbon dioxide (CO2) is the answer. High school science taught us that we breathe in oxygen and breathe out carbon dioxide. The truth is that we breathe out less oxygen than we breathed in, and more carbon dioxide than we breathed in. So what we see over time is an increase in the carbon dioxide levels in the air.

And our bodies don’t like that. As carbon dioxide levels rise we start to feel fatigued, nauseous, sleepy and then the head aches start. If it becomes too high then we will eventually pass out and die, but that is very unlikely in normal conditions.

So back to the buses. Our modern buses are equiped with great air conditioning and windows that can’t be opened. The air we breathe is totally controlled by the airconditioning and the bus companies. So what is the air quality like?

Recently I caught the M61 from Baulkham Hills to the city on a Saturday morning. It was 7:50 in the morning so the bus would have done a couple of trips already, but it was only half full. In theory this would have been an easy time to ensure good air quality.

The carbon dioxide levels were at 1,700 ppm (parts per million).

Fresh air is about 400ppm. Research has shown that people start to notice the impact when it reaches 1,000 ppm. By 1,500 ppm drowsiness and headaches will start to occur.

So on an early weekend morning the carbon dioxide levels were at 1,700 ppm. This means that people will be arriving at their destination feeling worse than when they left. If they are going to work, they will not be at their peak. If they are heading out for the day they will be tired before they even start.

It means that the drivers will be exposed to conditions that will be compromising their alertness. The drivers, however, have the advantage of being near the front door and will be exposed to the blasts of fresh air, just not on the final run into the city.

So next time you are on a bus and aren’t feeling the best, ask yourself if it is just yourself, or is the bus slowly poisoning you?

And for those who want to know the truth, I used a Keep Alert hand held CO2 monitor to check the bus levels.