Air Filtration Devices (AFDs), specifically those equipped with HEPA (High-Efficiency Particulate Air) filters, are integral tools in water damage restoration. Their utilization extends beyond simple dust removal, addressing critical air quality concerns that directly impact occupant health and the efficacy of the restoration process.
Why Restoration Contractors Use AFDs On Mold Damage Projects
Employee Health and Safety:
Mold remediation projects often disturb settled dust and debris, creating airborne particulates that can pose significant health risks to employees, even wearing respiratory protection.
Dust and Debris Control:
Restoration activities, such as demolition, material removal, and cleaning, generate substantial amounts of dust and debris.
AFDs effectively reduce the #s of these particulates, preventing them from spreading throughout the structure and becoming breathable.
This is especially important when dealing with materials that may contain irritants or allergens.
Contaminant Removal:
Mold remediation can introduce various contaminants into the air, including:
Fine particles from demoed building materials.
Microbial fragments associated with mold & bacteria.
HEPA filters are designed to capture particles at 0.3 microns with a high efficiency rate, effectively removing these contaminants from the air with 99.97% efficiency at this particle size.
How AFDs Work:
HEPA Filtration:
HEPA filters are composed of a dense network of fibers that trap airborne particles.
They achieve their high efficiency through a combination of interception, impaction, and diffusion as a means of trapping particulate.
They're tested against particle sizes of .3 microns. This particle size was selected by the CDC (Centers for Disease Control) & the AMA (American Medical Association) as being the deepest penetrating breathable particle type.Â
Airflow Management:
AFDs draw air through the HEPA filter, removing particulates before discharging clean air back into the environment.
Proper placement of AFDs is essential to maximize their effectiveness.
Creating negative air pressure by exhausting the filtered air outside of the structure, is often the best method.
Practical Considerations:
Filter Maintenance:
Regularly inspect and replace HEPA filters as the are either contaminated, or become loaded with particulate to ensure their optimal performance.
Clogged filters reduce airflow through the AFD.
HEPA filters must never be removed & replaced while in service on a project. In the event that it's required to replace the filter in the field, seal the unit up, take the unit outside, away from the project & perform the swap.
Unit Placement:
Position AFD intakes as close to where contaminants are, or where work is to be performed to capture airborne particulates at their source.
Consider using multiple AFDs in larger or heavily contaminated areas.
Determine the amount of air filtration required in CFM.
1. Determine the Volume of the Space:
Measure the Dimensions: Measure the length, width, and height of the room or area in feet. Â
Calculate the Volume: Multiply the length, width, and height to find the volume in cubic feet (ftÂł).
Example: An area that is 45 feet long, 36 feet wide, and 8 feet high has a volume of 45 ft x 36 ft x 8 ft = 12,960 ftÂł.
2. Calculate the Total Air Volume to be Changed:
Since we want 4 air changes per hour, we need to divide the volume of the room by 15 (or 1 air change every 15 minutes).
Example: 12,960 ftÂł / 15= 864. This means we need 864 cubic feet per minute to achieve 4 air changes per hour.
3. Calculate the Required Number of AFD Units:
Using the CFM rating of the AFD, we must divide the CFM required & divide it by the AFDs rating.
Example: You have available 500 CFM units. 864 / 500. This gives a value of 1.72.Â
Since this isn't a whole number we must round the unit up to 2.
Technicians would setup two 500 CFM AFDs in the property.
Practical Considerations:
Air Filtration Devices (AFDs): When using AFDs, their CFM rating indicates how much air they can move. Choose an AFD with a CFM rating that meets or exceeds the calculated requirement.
Be mindful that the CFM of the AFDs may be misadvertised by the manufacturer.
Some manufacturers will use a CFM rating without the HEPA filter in place. The amount of resistance to flow the HEPA filter creates can greatly minimize the actual CFM of the AFD.
This seems to be more prone to the larger AFD's vs the smaller 500 CFM units. For example a unit that is rated at 2000 CFM may be lucky to see 1600 CFM with the HEPA filter installed.
Attachment of ducting to the units can create a restriction that reduces overall airflow through the AFD.Â
Filter Changes:Â
The filters in AFDs should be swapped frequently to keep the airflow through the unit as high as possible. Loaded filters slow air down & can cause issues with the air changes per hour originally factored.
Prefilters:Â
Spun poly filter: should be swapped at the end of the day minimum.
This filter may require replacement during heavy dust generating activities such as using saws & sanders.
Pleated filters: should be swapped at the end of each day.
Primary/HEPA filter: This is generally replaced after 20-30 days of use, or after it's used on a project such as fire & smoke, or a sewage or trauma project where odors have permeated the primary filter.Â
There is unfortunately no decontaminating these filters. Ozone & hydroxyl will damage the adhesives that hold the fabric to the housing.
Additional Terminology:
Air Scrubber: this refers to an AFD that is set to neutral pressure.
Negative Air Machine: This refers to an AFD that's connected to the exterior of the building by ducting to draw the work area under negative pressure.
Module: Air Filtration Device (AFD) and Filter Training
I. Primary Filter (HEPA)
Function:
High Efficiency Particulate Air (HEPA) filters are 99.97% effective at arresting particles at 0.3 micron.
They trap ultra fine particulate in it's paper element.
 The final filter in a filtration system.
Operational Requirements:
Quality HEPA's must not allow air to bypass when as it becomes loaded with particulate.
HEPA's should ALWAYS be used when using an AFD, even when exhausting to areas where exposure is unlikely.
Alternative Operation:
If contractors decide to use an AFD without a primary filter, the unit should be dedicated to this purpose and be clearly marked as such.
Prefilters MUST always be used.
An AFD without a HEPA should NEVER be used in situations where the exhaust airflow could inadvertently affect any other structures or interact with humans.
Perhaps use a damaged AFD that fails a particle counter test for this application.
II. Validation of AFD and HEPA Filter Performance
Method:
Use a laser particle counter to compare air entering and exiting the AFD.
Example 1 (Acceptable):Â
Air entering AFD: 58,394 particles at 0.3 micron.
Air exiting AFD: 1,699 particles.
Calculation: 58,394 * 0.03 = 1750.47.
Conclusion: Filter operates at or above 99.97% efficiency.
Example 2 (Bypass):Â
Air entering AFD: 99,841 particles at 0.3 micron.
Air exiting AFD: 5,822 particles.
Calculation: 99,841 * 0.03 = 2995.23.
Conclusion: Filter is bypassing particulate.
Investigation of Bypassing Units:
Do not disassemble for the purpose of testing while inside an active project.
Remove the unit carefully and replace it with a verified functional unit.
Potential issues causing bypassing air:
A compromised seal between the filter cartridge and the AFD case.
A damaged case due to misuse or abuse.
Physical damage to the filter.
Used HEPA Filter Performance:
Undamaged, but used HEPA filters may show lower particle counts in the form of higher arrestance percentage than new filters.
By catching more & more particulate, the HEPA module will entrap more material.
Efficiency may surpass 99.97%, but CFM throughput is reduced as a consequence.
Low-Quality AFDs:
Cheaper AFDs may not provide an adequate seal between the HEPA filter and the AFD case.
Some units have little to no gasket material, resulting in bypassing contaminated air.
III. Particle Size and Filtration
.3 Micron Significance:
The healthcare industry identified particles at 0.3 micron as the MPPS (most penetrating particle size) for human inhalation.
Mold tends to be particles 10-15x larger than the test criteria.
IV. HEPA Filter Service Life
Filter Life Span:
It really depends on the particulate material being arrested, the particle counts, and the run time.
Specific Applications:
Smoke, fire, body decomposition, or sewage/bacteria: necessitate replacement upon project completion due to odor permeation and cross contamination risk.
Life on normal mold or water losses: Approximately 20 days of run time.
Airflow Reduction:
Operators will see a diminished airflow per day of operation.
After 20 days the airflow may be insufficient for the required Air Changes per Hour.
V. Pre-Filters
Function:
Protect and extend the life of the HEPA filter.
Trap particles at < 95% efficiency.
Must always be used with an AFD.
These filters should be swapped daily at the end of the workday after dust generating activities have ceased for the day.
Purpose: Knock down airborne particulate after demolition/dust generating activities.
Types:
Spun Poly filter: First filter, typically non-directional. Replaced at least once per day.
Pleated filter: Second filter, directional. Replaced once per day, potentially twice during high dust generation.
Ring Panel Filter: An optional filter that uses a metal ring around the perimeter to hold the filter in place without collapsing against airflow. This filter protects the HEPA from air bypassing clogged pre-filters. Generally replaced upon the projects completion.
Activated charcoal filter: Similar to a pleated air filter, but with odor absorption.
VI. Particle Trapping Mechanisms
Straining:
Particles larger than fiber openings are trapped.
Impaction:
Particles directly impact the filter medium.
Affects particles over 1 micron.
Interception:
Particles bump into the side of the filter medium.
Affects particles between 0.1 and 1 micron.
Diffusion:
Brownian motion causes very small particles to create a wavy pattern.
Affects particles smaller than 0.1 micron.
Electrostatic:
Negatively charged particles are attracted to positively charged fibers.
VII. MERV Rating
Definition:
Minimum Efficiency Reporting Value, established by ASHRAE.
Represents the lowest possible performance.
Indicates how many particles the filter will trap.
Higher the number, the more particles trapped.
HEPA has an equivalent MERV rating of 17 or higher.
VIII. Filter Thickness
Effect:
Filters with the same MERV rating trap similar amounts of particulate.
Thicker filters (e.g., 2") allow less pressure drop through the filter, resulting in more airflow.
Thicker filters contain more filter material surface area compared to the thinner filters of the same other dimensions.
Thicker filters also can often go longer between filter changes, relative to a thinner filter of the same MERV rating.
IX. Particle Arrestance
Misconception:
Particles larger than 0.3 micron are not necessarily 100% arrested.
Particles smaller than 0.3 micron do not necessarily pass through unobstructed.
Particles smaller than .3 micron typically have a higher arrestance rate relative to .3 micron particles.Â
Filter Material Structure:
The filter material is not a screen with evenly spaced holes.
It consists of random overlapping monofilament strings.
Material is composed of hundreds or thousands of layers.