Massachusetts Department of Public Health seal Massachusetts Environmental Public Health Tracking


Image of classroom hvac unit

In homes, apartments, schools, and businesses, ventilation brings in fresh air to the building, while removing stale air and odors. Ventilation is important for preventing the accumulation of normally occurring environmental pollutants, such as dust, gases, and moisture from building processes/activities, cleaning products, and building materials. Ventilation in a building can be natural sources, such as window, or mechanical systems.

What Type of Ventilation is Found in Homes?

Homes that are heated by furnaces and boilers or cooled with window air-conditioning units typically rely on natural ventilation from openable windows and doors, as well as leaks in the building, for fresh air. In single family or homes with two to three residential units, exhaust vents are typically the only sources of mechanical ventilation. Newer homes and updated older homes may have mechanical ventilation that filters air. Exhaust systems, particularly in bathrooms or kitchens, are commonly the type of mechanical ventilation found in most homes. Exhaust ventilation is important for removing common air pollutants to the outdoors. In buildings lacking mechanical ventilation, opening windows and reducing moisture and irritant sources is the best way to improve indoor air quality.

What Type of Ventilation is Found in Large Buildings?

Larger buildings, including housing with four or more units, public schools, municipal buildings, and commercial buildings, rely on heating, ventilating, and air-conditioning (HVAC) systems for filtering and distributing fresh air. HVAC systems in larger buildings include centralized air-handling units or in-room equipment such as unit ventilators found in schools and offices that supply fresh air and dilute normally occurring indoor air pollutants. These types of system also are also designed to return some air from the indoors and vent some air outdoors. Mechanical exhaust ventilation in these types of buildings helps to remove air, odors, and other pollutants.

The design, operation, and maintenance of a ventilation system is fundamental to maintaining good indoor air quality. A properly operating ventilation system can dilute and remove environmental pollutants. Periodic checking of the operation and maintenance of a mechanical ventilation system is important in making sure the system is operating properly.

What About Indoor Air Quality and Ventilation in Public Buildings?

Air sampling methods commonly used for understanding how well a mechanical ventilation system is working in high occupancy buildings include measurements for carbon dioxide, temperature, and relative humidity within an occupied building. The Indoor Air Quality Program measures these parameters during a school or public building assessment to gauge ventilation adequacy and comfort within rooms of a school or other high occupancy building.

Carbon Dioxide

Carbon dioxide is an odorless, colorless gas found naturally in the environment and in exhaled breath. Since carbon dioxide is generated by breathing air, increasing carbon dioxide levels within an occupied room suggests a lack of adequate ventilation. This is why carbon dioxide levels can be used as an indicator of the adequacy of a building's ventilation system. The Massachusetts Department of Public Health (MDPH) recommends that carbon dioxide levels be below the guideline level of 800 parts per million (ppm). Above this level, studies indicate increasing reports of headaches, lethargy, and irritation to the eyes and upper respiratory system.

During evaluations, Indoor Air Quality Program staff within the MDPH’s Bureau of Environmental Health take carbon dioxide measurements in every occupied accessible room or activity space. This is measured in high occupancy buildings so staff can determine if the ventilation is adequate for the types of activities that occur. These measurements, together with an examination of the ventilation equipment, help determine whether ventilation is adequate. Additional information regarding carbon dioxide can be found here.

Temperature and Relative Humidity

People naturally generate heat and moisture. These contribute to higher temperature and relative humidity, which can negatively affect occupant comfort and productivity. ‘Poor’ air quality complaints are often associated with temperature and humidity. For example, a room described as ‘stuffy’ may suggest that the temperature is too warm and/or indoor humidity is beyond a comfortable level. Temperature and relative humidity measurements in all accessible rooms help determine the adequacy of HVAC system(s) within a building.

Fluctuations of temperature in occupied spaces are common, even in a building with an adequate fresh air supply. However, the MDPH recommends that indoor air temperatures be maintained in the range of 70°F to 78°F to provide for the comfort of building occupants. Additionally, the MDPH recommends that relative humidity be maintained in a comfort range of 40 to 60 percent.

A sensation of dryness and irritation often occurs in a low relative humidity environment, which is a very common problem during the heating season in the northeast part of the United States. Low humidity environments contribute to upper respiratory and eye irritation. In the summer, indoor relative humidity levels are expected to be higher. Levels of relative humidity above 60 percent can contribute to feelings of heat-related discomfort.

This page was last edited on: