Protection against hazardous dust is a complex challenge. While dust often poses a respiratory or ingestion hazard, body protection is crucial to prevent contamination of underclothing and secondary exposure to others. Unlike liquids, dust particles float freely in the air and move with airflows. So how do they get inside protective clothing?
The answer is airflows and “The Bellows Effect.”
What is The Bellows Effect?
Watch a wearer of a disposable coverall as they walk, climb ladders, and go about their workday. Often, you’ll notice the legs, body, and arms repeatedly “ballooning,” as if being inflated and deflated. This happens because the fabric has little to no air permeability. Movement causes air to shift within the suit, being forced from one area to another because it cannot pass through the fabric. This results in constant blowing and drawing of air through any gaps in the suit—stitched seams, the zipper, the neckline, and cuffs—just like a pair of bellows drawing in and blowing out air. Consequently, particles in the surrounding air are drawn into the suit. While some may exit again, others catch on the wearer’s skin or clothing and remain inside.
This effect is most pronounced in disposable coveralls made from low or non-breathable fabrics, such as microporous film laminates (MicroMax NS) or flash-spun polyethylene, both of which have near-zero air permeability.
However, some disposable coveralls exhibit this effect less due to their fabric’s higher air permeability. These include garments made from SMS or SMMS polypropylene (where “S” stands for “Spunbonded” and “M” for “Meltblown,” indicating different layers in the fabric).
Consequences of The Bellows Effect
Both microporous film and flash-spun polyethylene fabrics offer high dust filtration efficiency—over 99% even for small particles—compared to SMS/SMMS fabrics. This suggests they should provide superior dust protection. However, the Bellows Effect can change this outcome.
Breathable SMMS fabric may be less effective at filtering particles, but its air permeability means air is not forced through other gaps like seam holes or zipper teeth, unlike non-breathable fabrics. The net result can be that total whole suit inward leakage is lower for an SMMS garment than for its non-breathable counterparts.
Is There Proof of The Bellows Effect?
Anecdotal evidence from a carbon black plant in England supports this. Carbon black, a fine particle used as a blackening agent (e.g., in tires), revealed the Bellows Effect in action. Workers wearing flash-spun polyethylene coveralls noticed that removing their suits after a shift left small black dots tracing the seams of their white shirts—evidence of particles drawn in through seam holes.
More concrete proof lies in the whole suit inward leakage test for EN Type 5 dust protective clothing:
- A test subject enters a cabin filled with fine dust particles.
- Three probes inside the suit (at the knee, lower back, and chest) measure particle penetration. A fourth probe outside provides a “challenge count.”
- The subject performs movements—standing, walking, and squatting—with rest periods between.
- 10 sample suits are tested across at least two test subjects, producing 90 inward leakage results.
Findings From the Type 5 Inward Leakage Test
Analysis of Safegard GP (breathable SMMS) and MicroMax NS (non-breathable microporous film laminate) coveralls revealed:
- Despite MicroMax NS offering superior particle filtration, overall inward leakage was lower for breathable Safegard GP (3.14% IL vs. 6.5% IL), proving the Bellows Effect reduces protection in non-breathable fabrics.
- Inward leakage increased with strenuous movements, particularly squatting. MicroMax NS exhibited over double the leakage compared to Safegard GP (13.6% IL vs. 6.3% IL).
- Movement significantly impacts inward particle draw-in; strenuous work increases the effect, particularly in non-breathable fabrics.
- The zipper front fastening is a major weakness for dust protection. Probe counts showed high particle penetration through the zipper. For MicroMax NS, sealing the zipper with tape significantly reduced inward leakage (0.27% IL vs. 6.2% IL).
Key Takeaways
- The Bellows Effect is real and impacts protection. A superior particle filtration fabric may not provide better protection if it has low breathability.
- Non-breathable fabrics require additional protective measures, such as taping the zipper and PPE joins or using a coverall with sealed seams.
- Breathable fabrics may offer better overall dust protection by minimizing the Bellows Effect.
