Negative pressure, HEPA air filtration and air changes

In plain English

Containment works best when air leaks into the work area rather than out of it. R520 calls for keeping contaminated areas under negative pressure relative to clean areas, typically with HEPA air filtration devices (AFDs) exhausting air as negative air machines. The commonly adopted target is a differential of at least 5 pascals (0.02 inches of water), with some jobs needing about 7. Pressure is checked with a manometer, smoke or the inward bow of the plastic, and sensitive sites such as clinics or daycares warrant alarmed, logging monitors.

For dilution, R520 cites the industrial hygiene minimum of four air changes per hour. Required airflow equals room volume times air changes, divided by 60. It notes that an AFD's rated airflow is higher than what it actually delivers once filters load and ducting adds resistance. AFDs use pre-filters and a secondary filter to protect the HEPA stage. Air scrubbers that only recirculate air inside a space can push it positive and should be watched with a particle counter.

Moving that much air has risks. Negative pressure can backdraft gas appliances, so R520 asks for carbon monoxide detectors and shutting down affected gas appliances. In humid weather it can also pull damp outdoor air in. When shutting down an AFD, seal the intake first to keep collected spores inside.

Key points

• Keep the work area at about -5 Pa (0.02 in. w.c.) or lower.
• Aim for at least four air changes per hour.
• Real airflow is lower than the machine's rating.
• Use CO detectors; disable gas appliances in the affected zone.
• Seal the AFD intake before switching it off.

Official text

This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.

Citations

• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 11, pp. 113–115. Best practice
• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 5, p. 53. Best practice
• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 5, pp. 56–57. Best practice

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