Dosage, Cube, and a Half-Loss Time That Barely Passes
A two-story, 2,200 ft2 single-family house is scheduled for sulfuryl fluoride fumigation for drywood termites confirmed in the attic joists and in the mudsill at the crawlspace. Living-space ceiling height is 8 ft; the attic averages 4 ft of clearance; the crawlspace is 2 ft deep. After 24 hours of equilibration the minimum wood temperature recorded in the attic is 62 F, which the label's dosage table converts to a 1.5X factor (1X = 10 oz per 1,000 ft3). Company SOP requires a documented half-loss time (HLT) of at least 2 hours on every structural job. After the charge, with the circulation fan running, the fumer records decay at the living-room sampling point: 100% of target at t = 0, 82% at 30 minutes, 67% at 60 minutes. Both attic and crawlspace are inside the tarp envelope.
1. Calculate the tarped volume, the label rate, and the total product to be introduced. Show the arithmetic and state the result in pounds and ounces.
Read the worked answer
Volume (cube), by component rather than footprint alone:
- Living space: 2,200 ft2 x 8 ft = 17,600 ft3
- Attic: 2,200 ft2 x 4 ft = 8,800 ft3
- Crawlspace: 2,200 ft2 x 2 ft = 4,400 ft3
- Total = 30,800 ft3 = 30.8 units of 1,000 ft3
Rate: the label table, driven by the 62 F minimum wood temperature, gives 1.5X; with 1X = 10 oz/1,000 ft3 that is 15 oz per 1,000 ft3.
Dose: 30.8 x 15 oz = 462 oz. At 16 oz/lb: 462 / 16 = 28.875 lb = 28 lb 14 oz.
Judgment points the examiner is looking for: use the coldest wood temperature, not ambient air, because kill rate falls with temperature; include every tarped void that holds wood or connects to infested wood, because an uncalculated attic is an underdosed attic; and never round the dose down to the nearest full pound -- round up or meter exactly.
2. Compute the half-loss time from the decay data and decide whether the charge may be allowed to run as dosed, given the SOP and the label.
Read the worked answer
The decay is exponential, so use the ratio between successive readings.
ln(0.5) / ln(0.82) = (-0.693) / (-0.1985) = 3.49 half-lives per 30-minute interval
HLT = 30 min x 3.49 = about 105 minutes, i.e. roughly 1.75 hours
Check with the 60-minute figure: 0.82^2 = 0.67, which matches the recorded 67%, so the data are internally consistent and the estimate is trustworthy.
Decision: 1.75 h is below the 2-hour SOP minimum, even if it may sit above the label's floor. An HLT that short means the envelope is leaking -- typically ridge vents, gable vents, the chimney, or tarp at grade -- and the concentration at the pest will fall faster than planned, so the target Ct will not be met at the extremities of the structure.
Action: do not simply let it run and hope. Identify and seal the leaks, then either re-charge to the label's higher dosage for a short half-loss time or abort, re-seal, and re-fumigate. Document the readings, the corrective action, and the basis for the final decision. Note that the clock restarts after any re-charge.
3. The technician proposes to take the half-loss readings only at the living-room wall thermostat and to skip the attic and crawlspace because 'the living room is where the people will be.' Explain why that is or is not acceptable and what the reading location must represent.
Read the worked answer
Not acceptable. The living room is the least useful place to measure on this job.
- The pest is in the attic and the mudsill. The concentration that matters is the concentration at the target, for the full exposure period. A comfortable living-room curve says nothing about either.
- Leak paths are not evenly distributed. Ridge vents, gable vents, and the chimney sit in the attic; the crawlspace has foundation vents and a soil surface. Attic and crawlspace curves typically fall away faster than the conditioned space.
- Density matters. Sulfuryl fluoride is roughly 3.5 times as dense as air, so it pools low and stratifies; a single point at 4 ft in one room can read high while a closet or crawlspace reads higher, or the reverse during aeration.
- The half-loss time is a property of the envelope, not of one room. It must be calculated from the worst-case (fastest-decaying) location, because that location determines when the pest exposure drops below the required Ct.
Correct practice: multiple fixed sample points, including the attic and crawlspace, sampled through the same tubing and logged at the same time intervals so the curves are comparable.