Every year, thousands of employees suffer respiratory irritations, disease and sometimes death due to their lack of understanding regarding ventilation types. And one of the biggest issues is knowing that mechanical ventilation is local exhaust, but local exhaust is not mechanical ventilation. It’s like understanding that a tomato is a fruit, but you never put the tomato in the fruit salad.
Let’s start at the beginning. There’s a gap between Fed OSHA, Cal-OSHA, ASHRAE and chemical manufactures. It’s this gap that creates the problem for the average American worker. See ASHRAE does define mechanical ventilation, but they don’t define local exhaust. OSHA defines local exhaust but they don’t define mechanical ventilation. And chemical manufactures or at least the authors of those wonderful MSDS (now just SDS) don’t define any of those terms, but sure like to use all the terms. The result can be deadly.
General ventilation is the opening of doors and windows to create air flow thru a building or space. This is typically what you do in your home during the spring and fall months. General ventilation allows chemicals to permeate the air and dilute naturally. This means that
Mechanical ventilation takes general ventilation a step further and adds a fan to the mix to circulate air. This creates an artificial air flow to help move the chemicals out of the space faster and with more certainty.
Local exhaust captures the chemical prior to dilution in the air and removes it from the environment. Local exhaust requires 5 parts. As OSHA describes it “A typical local exhaust ventilation system is composed of five parts: fans, hoods, ducts, air cleaners, and stacks. Local exhaust ventilation is designed to capture an emitted contaminant at or near its source, before the contaminant has a chance to disperse into the workplace air.”
Know the Difference
There is a huge difference between mechanical and local ventilation and knowing this difference can save your life. When an SDS calls for local exhaust you cannot use mechanical ventilation, you must have all 5 parts of the local exhaust system. When an SDS does call for mechanical ventilation, you can use local exhaust because it is more effective than mechanical. Don’t confuse the two and you will live to breathe fresh air another day.