Which of the following may limit the effectiveness of an oxygen monitor?

The effectiveness of an oxygen monitor can indeed be limited by changes in elevation. As elevation increases, the atmospheric pressure decreases, which leads to a lower concentration of oxygen in the air. This can result in readings that may not accurately reflect the true oxygen levels in a given environment. For example, an oxygen monitor calibrated for sea level may indicate that oxygen levels are safe, while in reality, at higher elevations, the reduced oxygen concentration could pose a health risk.

While other factors such as the presence of flammable vapors, humidity control, and extreme heat can also affect various types of detection devices, they do not primarily impact the measurement of oxygen itself. Flammable vapors might interfere with certain types of gas detection technologies but are more relevant to the monitoring of combustible gases. Humidity can affect the performance of some instruments by causing condensation which may lead to inaccurate readings, particularly for electronic sensors, but it does not specifically relate to the direct measure of oxygen concentration in the same way that changes in elevation do. Extreme heat can also impact equipment performance, but typically, oxygen monitors are designed to function within a specific temperature range. Thus, among the provided choices, changes in elevation stand out as a key factor that limits the effectiveness of an oxygen monitor.