Tarpaulin Placement, Gas Introduction, and Concentration Monitoring
Objective. Students will describe and sequence the steps for tarpauling a structure, gas introduction using cylinder equipment, and in-structure concentration monitoring with a gas detection instrument, explaining how readings are used to confirm lethal CT is achieved.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.
VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.
Structural Fumigation: Tarpauling, Gas Introduction, and Concentration Monitoring
Structural fumigation is one of the most regulated and hazardous procedures in the pest control trade. Every step must be performed in the correct sequence and with strict attention to safety. This lesson walks through tarpaulin placement, gas introduction using cylinder equipment, and in-structure concentration monitoring using gas detection instruments.
Tarpaulin Placement
The tarpaulin ("tarp") confines the fumigant gas within the structure long enough to achieve a lethal concentration. Common tarp materials include vinyl-coated polyethylene and nylon-reinforced polyethylene sheets designed to be gas-retentive.
Placement follows a clear sequence:
1. Pre-fumigation inspection — Confirm the structure is cleared of all people, pets, plants, and unprotected food items. All occupants must have vacated before tarping begins.
2. Drape the tarp — Teams deploy the tarp over the roof and down all four sides, reaching the foundation perimeter. Multiple panels are required on most structures; panels must overlap by a minimum distance specified on the fumigant label (commonly at least 12 inches, though always defer to the label).
3. Seam sealing — Overlapping edges are sealed by folding and weighting or by using magnets or seam clamps designed for fumigation use. Gaps at seams are a primary cause of concentration loss and failed treatments.
4. Foundation seal — Sand snakes (fabric tubes filled with sand) or other weighted seals are placed along the entire foundation perimeter to prevent gas from escaping at ground level. This is the second most common failure point after seam gaps.
5. Probe tube placement — Before the tarp is fully sealed, monitoring probe tubes are fed through the tarp at pre-planned locations inside the structure. Probes are placed at different heights and in areas where gas distribution may be reduced (under floors, in dense contents). These tubes allow gas readings without breaching the tarp during fumigation.
After sealing, the fumigator-in-charge performs a walk-around to confirm all seal points are secure.
Gas Introduction Using Cylinder Equipment
The most widely used structural fumigant in the United States is sulfuryl fluoride (sold under trade names). It is supplied in pressurized cylinders and introduced through a manifold system.
- Cylinder manifold connection — Cylinders are connected to a hose manifold that routes gas through a probe inserted under the tarp. Connections must be leak-checked before opening cylinder valves.
- Label-directed application rate — The amount of gas introduced is calculated from the structure volume (cubic feet) and the target dose from the fumigant label. Label rates vary by target pest and are expressed in ounces of fumigant per thousand cubic feet (oz/1,000 ft³). Always follow the label; it is the law.
- Gas introduction location — Gas is typically introduced low in the structure (at or near floor level) to allow it to distribute upward. The probe hose end is positioned away from vents or concentrated openings.
- Personal protective equipment (PPE) — During gas introduction, all personnel must wear the appropriate supplied-air respirator (SCBA or airline respirator) specified on the label and by OSHA standards. A second trained person must be present; no solo fumigation work is permitted during gas introduction.
Concentration Monitoring and CT Values
Monitoring confirms that a lethal concentration is maintained throughout the exposure period. The key concept is CT value.
CT (concentration × time) is expressed in ounce-hours per thousand cubic feet (oz-h/1,000 ft³) or in ppm-hours, depending on the label. A required CT is reached when the measured concentration (C) multiplied by the cumulative exposure time (T) meets or exceeds the label minimum for the target pest at the measured temperature.
Temperature matters because gas activity decreases in colder conditions. Label dosage tables list minimum CT requirements at different temperature ranges. The fumigator reads the lowest temperature in the structure during the exposure period and uses that value to find the required CT.
Instruments Used
- Colorimetric detector tube pump — A hand-operated bellows or piston pump draws air through a graduated glass tube containing a reagent that changes color in proportion to fumigant concentration. Readings are taken by matching the color change length to the scale printed on the tube. Tubes are fumigant-specific and have a defined shelf life.
- Electronic fumigant detector — A portable electronic instrument with a sensor that provides a direct digital readout of concentration, often in parts per million (ppm). These instruments require regular calibration and bump testing before use.
Readings are taken at each probe tube location at scheduled intervals (for example, at 2 hours, midpoint, and before clearance). The lowest reading among all monitoring points is used for CT calculations — the weakest point in the structure sets the standard.
Confirming Lethal CT
The fumigator records each concentration reading and the time it was taken. Using the trapezoidal method (averaging consecutive readings and multiplying by the time interval), cumulative CT is calculated. When cumulative CT at the lowest monitoring point meets or exceeds the label minimum for the pest and temperature band, the required exposure is confirmed.
If CT is not met, exposure time is extended and monitoring continues. The fumigator does not proceed to aeration until CT is confirmed.
Safety Reminders
- Never enter a fumigated structure without a calibrated gas detector and appropriate respiratory protection. Entry below label-specified clearance concentrations requires supplied-air equipment.
- Post all required warning signs and lock or seal all entry points before gas introduction.
- Have emergency response information (Safety Data Sheet, poison control contact) immediately accessible on site.
- Cylinder handling follows compressed gas safety practices: secure cylinders upright, use appropriate wrenches, never force connections.
Key terms10
Worked examples4
- Step 1 — Calculate CT for the first interval (hours 0–4): 3.0 oz/1,000 ft³ × 4 h = 12.0 oz-h/1,000 ft³
- Step 2 — Calculate CT for the second interval (hours 4–8): 2.5 oz/1,000 ft³ × 4 h = 10.0 oz-h/1,000 ft³
- Step 3 — Sum the two intervals: 12.0 + 10.0 = 22.0 oz-h/1,000 ft³
- Step 4 — Compare cumulative CT to the label minimum: 22.0 oz-h/1,000 ft³ < 60 oz-h/1,000 ft³ required. CT has NOT been met; exposure must continue.
- Step 1 — Convert structure volume to thousands of cubic feet: 18,000 ÷ 1,000 = 18 units
- Step 2 — Multiply by the label rate: 18 × 4 oz = 72 oz
- Step 1 — Interval 1: hours 0–2. Only the hour-2 reading is available; assume starting concentration is 0 at hour 0 (gas just introduced). Average = (0 + 4.0) ÷ 2 = 2.0 oz/1,000 ft³. Duration = 2 h. CT₁ = 2.0 × 2 = 4.0 oz-h/1,000 ft³
- Step 2 — Interval 2: hours 2–6. Average = (4.0 + 3.5) ÷ 2 = 3.75 oz/1,000 ft³. Duration = 4 h. CT₂ = 3.75 × 4 = 15.0 oz-h/1,000 ft³
- Step 3 — Interval 3: hours 6–10. Average = (3.5 + 3.0) ÷ 2 = 3.25 oz/1,000 ft³. Duration = 4 h. CT₃ = 3.25 × 4 = 13.0 oz-h/1,000 ft³
- Step 4 — Sum all intervals: 4.0 + 15.0 + 13.0 = 32.0 oz-h/1,000 ft³
- Step 1 — Determine remaining CT needed: 48.0 − 32.0 = 16.0 oz-h/1,000 ft³
- Step 2 — Divide remaining CT by current concentration: 16.0 ÷ 3.0 = 5.33 hours
- Step 3 — Round up to a practical monitoring interval (typically the fumigator extends to the next monitoring check). At minimum, approximately 5.3 additional hours are needed if concentration remains at 3.0 oz/1,000 ft³.
The math in this lesson2
- MA7 1 · Expressions and Equations — Variables and expressions A-SSE.1
- MA7 2 · Expressions and Equations — Order of operations A-SSE.1
- BZ5 5 · Money at Work — Gross pay and overtime N-Q.1
- BZ5 9 · Interest, Credit and Saving — Simple interest A-SSE.1
- BZ5 17 · The Small Business Ledger — The monthly budget N-Q.1
- BZ5 20 · The Small Business Ledger — Inventory and cost of goods sold N-Q.1
Materials: Training structure prop or scaled model frame, tarpaulin material samples, sand snakes or weighted seal props, probe tube samples, colorimetric detector tube pump and tubes (or simulation trainer), electronic fumigant detector (demonstration unit), fumigant label dosage and temperature tables, field notebooks, laminated step-sequencing cards
Check: Field notebook reviewed by instructor: student must correctly record the sequential steps of tarpauling and monitoring and show a correct CT calculation for a provided scenario (temperature, target pest, structure volume) with no errors in the label table look-up; 80% of notebook entries must be accurate and all safety-critical steps must be present.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
