Professional air testing may be needed to determine the extent of a building's indoor air quality problems. An indoor air consultant may recommend a carbon dioxide test as an initial indicator of sick building syndrome. Note that this is not a carbon monoxide test, but rather a test for the natural by-product of breathing, carbon dioxide.
The reason this test can indicate sick building syndrome? An excess of CO2 indicates that there is a definite lack of fresh outside air coming into the building. The exhaled air remains trapped inside, the result being stale or even stagnant air. A Carbon dioxide test may reveal a further investigation is needed, depending on the results.