Adhesive and Sealant Vapors in Tight Spaces

Why this matters

A can of PVC cement or contact adhesive is used in open air every day without trouble, so crews stop thinking of it as a hazard. Put the same can in a crawlspace, a vanity cabinet or a windowless half bath, and two things happen together: the vapor builds up toward a concentration that makes people dizzy and sick, and it builds up toward a concentration that burns. Those two hazards share one cause and one cure, and they have to be managed together, because the fan you set up for the first can become the ignition source for the second.

The products that cause the trouble

Solvent cements and primers for PVC, CPVC and ABS pipe work by dissolving the pipe surface. Their solvents commonly include tetrahydrofuran (THF), methyl ethyl ketone (MEK), cyclohexanone and acetone. Primers are mostly solvent with a dye. Both are highly flammable liquids with flash points well below room temperature.

Solvent-based contact adhesives, used for laminate, rubber membrane, carpet and trim, are spread over large areas and left to flash off before the surfaces are joined. That design means the entire solvent load is meant to evaporate into the room. Formulas have used hexane, heptane, toluene and acetone among others. n-Hexane deserves a specific note: repeated exposure causes peripheral neuropathy, nerve damage starting as numbness and tingling in the hands and feet, and outbreaks among workers using hexane-based contact cement in poorly ventilated spaces are well documented.

Solvent-based construction adhesives and some sealants release smaller amounts but over a longer cure. Acetoxy-cure silicone releases acetic acid (the vinegar smell), an irritant but not a fire concern at normal quantities. Spray foam and other isocyanate products are a separate hazard with their own card; see related: Isocyanates and Spray Foam in the Homes You Enter.

Water-based formulas exist for most of these uses, and where they are acceptable for the application they remove most of both hazards. That substitution question belongs to control selection; see related: The Hierarchy of Controls on a Service Call.

Two hazards from one vapor

Inhalation. Most of these solvents are central nervous system depressants at high concentration: headache, dizziness, nausea, slowed reaction, then drowsiness. In a tight space the first symptom can be the one that matters, because a dizzy tech in a crawlspace is a fall, a cut or a burned hand waiting to happen, and impaired judgment is itself a symptom. Several also irritate eyes and airways, and some (hexane, MEK combined with hexane) carry chronic effects. The solvents reference covers the individual chemicals and their limits; see related: Common Solvents + Cleaning Agents Reference.

Flammability. Every flammable vapor has a lower explosive limit (LEL), also called lower flammable limit: the lowest concentration in air that will ignite. For these solvents it is roughly 1 to 2.5 percent by volume. That sounds high, but it is reached far more easily than people expect in a space with little air movement, because:

• The vapors are heavier than air. They flow down and pool at floor level, in low corners, in a crawlspace sump, in the bottom of a vanity cabinet, rather than mixing evenly into the room.
• Large surfaces evaporate fast. An open can, a dauber laid on the ground and a freshly coated joint all emit at once, and contact adhesive spread over a countertop is a large evaporating surface by design.

A space with vapor building toward the LEL usually smells strongly well before it gets there, but odor is never the gauge. People adapt to a solvent smell within minutes, and the smell says nothing about how close the low point is to ignition.

Where ignition comes from in a house

Ignition is the half that crews underrate. In a crawlspace or utility area the common sources are:

• Standing pilots and burners: a gas water heater, furnace or boiler, often in the same crawlspace or basement where drain work happens. A pilot near the floor is in the worst possible place, right where vapor pools.
• Motors that cycle on their own: sump pumps, well pumps, condensate pumps, refrigerator compressors, the furnace blower.
• Your own equipment: the work light, the extension cord connection, a drill, a portable fan whose motor is not rated for flammable atmospheres.
• Light switches and plug-ins thrown while the space is full of vapor.
• Static and smoking.

Construction flammable-liquid rules address this directly: 29 CFR 1926.152 for construction work and 29 CFR 1910.106 for general industry govern storage and use of flammable liquids, including keeping ignition sources out of areas where vapors may be present.

The ventilation layout that works

The rule is: move air through the low point, and keep every motor and switch out of the vapor path.

  outside                       crawlspace
           vent A                              vent B
  [fan]===>|  fresh air -->  -->  -->   -->  -->|---> out
  (motor   |                                    |
  outside) |    joints     cement can     pilot |
           |    here       (capped)      (off)  |
  ---------+-----------vapor pools at grade-----+------
           ^ fan blows IN so its motor sits in    ^
             clean air, never in the vapor         exit vent

Two points that diagram is showing. Blowing clean air in from outside, with the fan's motor in the outside air, pushes vapor toward and out of a second opening; an ordinary fan pulling vapor out would draw the flammable mixture straight through its own motor. And the air path should sweep across the floor and the work, because that is where the vapor is. If a space has only one opening, dilution is weak and the job is closer to a confined-space question than a ventilation one.

Measuring instead of guessing

A multi-gas meter with an LEL sensor reads the flammable atmosphere as a percentage of the LEL, not of the air. That scale is the useful one: 10 percent LEL means the space holds one tenth of the vapor needed to ignite. OSHA's permit-required confined space standard treats an atmosphere at or above 10 percent of the lower flammable limit as a hazardous atmosphere (29 CFR 1910.146 for general industry; 29 CFR 1926 Subpart AA for construction), and that 10 percent figure is the common stop point for work with any ignition possible. Take readings at the floor and low points, where vapor sits.

Note that LEL sensors are calibrated to a reference gas, often methane or pentane, and read other vapors with a correction factor the manufacturer publishes. A sensor calibrated to methane can read solvent vapor low, which runs in the unsafe direction; check the correction factor for the solvent you are using before trusting a number near the stop point.

Worked example: a drain repair in a 24-inch crawlspace

A plumber is replacing 20 feet of 3-inch PVC drain under a house with about 24 inches of clearance, cutting in a new wye and making 8 solvent-welded joints, each primed and cemented. Access is one hatch on the north side; there are two foundation vents on the east and west walls. A gas water heater with a standing pilot sits in the crawlspace about 12 feet from the work.

Ignition first. Before any can is opened, the plumber asks the customer to let him turn the water heater's gas valve to "pilot off" for the duration, explaining that cement vapor settles along the crawlspace floor and the pilot sits about 6 inches above it. He also confirms the furnace is upstairs and the sump pump is in the basement on the other side of a foundation wall. The work light is a sealed LED on a cord plugged in outside the hatch, so no connection sits in the crawlspace.

Ventilation. The fan goes outside the east vent blowing in, with the west vent opened as the exit, so the air path runs across the work area. The hatch stays open as the plumber's way out.

Minimize the source. He dry-fits all 8 joints first, then opens the primer and cement only for the welding, keeps each can capped between joints, and lays the daubers on a rag in a sealable bag rather than on the soil.

Meter. His meter, calibrated on pentane, reads 0 percent LEL at the floor before cementing. Midway through the 8 joints it reads 4 percent LEL near the cement can. That is under the 10 percent stop, and with the fan running and cans capped it stays there. Had it reached 10 percent, the stop is: cap everything, get out, let the fan run until the floor reading drops back toward zero, and find out why the airflow was not doing its job before resuming.

Inhalation check. After about 20 minutes he has a mild headache. That is the solvent, and it is a signal the dose at his face is higher than the LEL reading alone suggests, because the meter is reading flammability, not the far lower exposure limit for health. He climbs out, takes ten minutes in fresh air, and finishes the remaining joints with an organic vapor cartridge respirator under the shop's written program per 29 CFR 1910.134.

Restoring the pilot, with its check. After the last joint, he leaves the fan running for 30 minutes, then takes a floor reading at the water heater: 0 percent LEL. Only then does he relight the pilot following the manufacturer's lighting instructions on the appliance label, and he takes the can, the bagged daubers and the rags out of the crawlspace before he does.

What would change the call. If the only access were the hatch with no second opening, cross-ventilation would not work, and the space starts to look like a permit-required confined space: that is the point to stop and use the shop's confined space procedure, not this card. If the pilot could not be shut off because the customer refused, the cementing moves as far from the appliance as the job allows and the meter becomes the controlling instrument, with the work stopped at the first reading that climbs toward 10 percent.

The failure mode. A fan set inside the crawlspace pulling air out, its motor sitting in the pooled vapor, beside a water heater pilot nobody thought to ask about. The plumber gets a headache and blames the heat.

References

• OSHA 29 CFR 1926.152 and 29 CFR 1910.106, flammable liquids
• OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA, confined spaces (10 percent LFL hazardous atmosphere); 29 CFR 1910.134, respiratory protection
• NIOSH Pocket Guide to Chemical Hazards, entries for n-hexane, tetrahydrofuran, methyl ethyl ketone and cyclohexanone
• See related: Common Solvents + Cleaning Agents Reference; Confined Space Entry (OSHA 1910.146); How to Ventilate a Small Space You Are Working In
• See related: The Hierarchy of Controls on a Service Call