Making jobsite ventilation a breeze
On the surface, ventilation is a simple concept. When we want more fresh air, we can open a window or a vent, or turn on a fan. At worksites, proper ventilation is one of the most important controls that can be used. In simple terms, it’s a way to control the quality of the work environment using flowing air, whether that’s to remove, replace, or refresh the current atmosphere.
Ventilation is used to remove airborne contaminants such as fumes and replace the contaminated air with fresh (make-up) air to control occupant exposure to the contaminants, reduce the potential for fire or explosion, and ensure the environment is safe and healthy. The make-up air may be filtered, heated, cooled, humidified or treated to meet the needs of the occupants.
You’ll find ventilation in most structures, if only to provide comfort for the occupants. In workplace settings, ventilation is typically required for tasks that generate vapors or fumes, or that result in the creation of dust.
Two basic types
The ventilation used in in workplaces falls into one of two types: general or local. General ventilation (sometimes referred to as dilution ventilation) uses fresh, clean air to reduce the concentration of airborne contaminants or adjust the temperature of the space. When you open a window or door to clear the air, or point a cooling fan at the worksite, you’re using general ventilation.
General ventilation is an acceptable choice in situations where the amount and movement of the air being introduced is enough to keep the concentration of contaminants at safe or acceptable amounts, when the contaminants are of low toxicity and flammability, and when the materials lack properties that may damage the space or cause discomfort to the workers.
In contrast, local ventilation systems are used to keep contaminants within a limited area, transfer those contaminants outside the space, and sometimes, to introduce fresh air to the area. Fume hoods, downdraft workstations, and spray booths are examples of local ventilation. In some cases, local ventilation systems have filters designed to capture the contaminants, so they don’t exhaust into the outside air. By enclosing the source of contaminants and limiting the area being treated, local ventilation reduces the impact on space conditioning costs.
Providing make-up air
While it’s easy to concentrate on the part of systems designed to remove contaminants, an equal amount of attention should be given to the process of replacing the air that has been removed. If more air is exhausted than is replaced, negative pressure will be created in the space, which can interfere with worker comfort and well-being. Negative pressure also increases air infiltration through cracks and small openings, which can negatively affect space conditioning costs.
Two simple tests can be used to determine whether a room is experiencing negative pressure. One is to test doors that open to the outside. If it’s difficult to open the door, it may be a sign that the outside pressure is higher than what’s in the space. You can also crack the door open and use a smoke tube. If the smoke goes through the door, there is positive pressure in the room. If the smoke moves away from the door toward the interior room, it’s a sign of negative pressure and a need to bring more outside air into the space.
Making the choice
Deciding which type of ventilation should be used depends upon the space, the tasks being performed there, and the impact those tasks have upon workers and the space itself.
General ventilation is generally a less costly approach that entails less maintenance. It works well with combustible gases and vapors, and it can also be used when small amounts of chemical with low toxicity are present. However, it’s not the right choice for highly toxic substances, or for operations involving dusts or fumes generated by metalwork. In addition, because it involves larger spaces and volumes of air, it impacts heating and cooling costs.
For operations where general ventilation falls short, local ventilation provides the necessary performance. While it usually costs more to design and install and demands more maintenance, that’s offset to some degree by the fact that it requires less make-up air to be heated or cooled. Local ventilation is the best choice for contaminants that are highly toxic, as well as for dusts and fumes that cannot be controlled with general ventilation.