How can the freshness of the air in a room be measured?

The freshness of the air in a room can be measured using an anemometer, which is an instrument designed to measure air velocity. By assessing how quickly air is moving, it can also give indications about the ventilation of a space. Good air circulation is often associated with fresh air, while stagnant air can suggest a buildup of carbon dioxide and other pollutants, indicating that the air may not be fresh.

While other instruments have their own specific uses, such as thermometers for temperature, barometers for atmospheric pressure, and hygrometers for humidity levels, none of these directly address air freshness in terms of circulation and ventilation. The anemometer's primary role in measuring air movement makes it the most relevant tool for assessing the freshness of the air in a room.