Negative Pressure Guidelines
What do I need to do to establish negative pressure?
To establish adequate negative pressure in a containment, you must have:
- at least 4 air changes every hour;
- a negative pressure of -0.02 inches of water column (" WC); and
- a recording manometer that
- must be checked every 2 hours
- must be zeroed before work begins each day
- must be calibrated at least once a year
What if the recording manometer breaks down during a project?
If your recording manometer breaks down during a project you must:
- replace it within 24 hours; or
- repair it within 24 hours
You must also:
- document hourly pressure readings
- document the failure and your actions in your site log and have that log available at the work site.
Remember that your manometer must be operational at the start of the project in order to use these options. Arriving at the job site with a manometer that is out of printer paper or ink does not mean you may immediately start documenting hourly pressure readings.
I can’t achieve -0.02" WC. Can I use the "6 Air Change" alternative?
Before you use the "6 Air Changes" alternative, make sure you have checked all of the following items:
- all critical barriers are in place.
- all critical barriers are properly sealed.
- test critical barriers by switching on the negative air machines and measuring the containment negative pressure before putting poly sheeting on the floor and walls
- wall and floor sheeting in place and sealed.
If you have done all of these items and documented this, and you are still unable to achieve -0.02" WC, then you may use the "6 Air Changes" alternative.
What is the "6 Air Changes" alternative?
If you have completed all of the previous steps and are still not able to achieve or maintain -0.02" WC, you must:
- Maintain as close to -0.02" WC as possible.
- Increase the number of air changes to at least 6 air changes per hour.
You must also document:
- The probable cause of failure to establish or maintain -0.02" WC.
- The dates and times of the failure.
- The name of the site supervisor in charge at the time.
If you are able to achieve -0.02" WC, document how you did so.
Where do I exhaust my negative pressure system?
The negative air machines that make up part of your negative pressure system must be exhausted to the outside of the building. If it isimpossible to exhaust to the outside, the machine can be exhausted indoors if:
- the exhaust is sampled with at least 1 air sample every four hours during abatement work.
You must also document:
- the reason why it cannot be exhausted outside, and
- the date, time and the supervisor in charge of the work site.
What kind of records do I need to keep?
The following documentation must be maintained for 30 years after completion of the project:
- The negative pressure measurements must clearly indicate the pressure measurements
- the date of the measurement
- time of the measurement
- the containment to which the measurement applies
- all instances of negative pressure deviation from the minimum requirements must have a written explanation. Write a brief description on the manometer tape and a detailed description in your daily site log.
How do I find out more information?
For more information about asbestos, contact the Asbestos Program at MDH through the internet or by telephone at (651) 201-4620.