What REI actually is — and what it isn't
Re-entry interval is the time specified on the PMRA product label between when a pesticide is applied to a surface and when people may safely re-enter that area. The interval exists because freshly applied liquid formulations off-gas solvent carriers and break down from their peak surface concentration during the first minutes to hours after application. What REI is not: it's not the 'danger window,' meaning re-entry at REI+1 minute is not safe while REI−1 minute is dangerous. REIs are regulatory minimums built with margins — typically 10× the dose at which no effect is detectable in the most sensitive study population. The practical implication is that brief exposure slightly before REI completion is not an emergency for healthy adults. What the margin does not cover: people with asthma, chemical sensitivity, pregnancy, or young children (particularly crawling infants). These populations have lower exposure thresholds and should observe the full REI — or an extended version where we recommend it.
How PMRA sets residential REIs
PMRA determines REI through a process combining toxicological data and exposure modelling: 1. The manufacturer submits acute and subchronic toxicology studies identifying the no-observed-adverse-effect level (NOAEL) — the dose at which no harm is detectable in the most sensitive species tested. 2. PMRA applies a 10× safety factor for human extrapolation from animal data (inter-species factor), plus a further 10× factor for intra-species variation (accounting for children, sensitive individuals). The resulting 100× safety margin below the NOAEL establishes the residential acceptable daily intake. 3. Exposure modelling determines how quickly surface residue off-gasses and binds to substrate under typical residential ventilation conditions. The REI is set at the time when estimated exposure from re-entry drops below the acceptable daily intake with the safety margins applied. For most modern residential products, this results in 1-hour REIs for crack-and-crevice interior applications — the product binds to substrate quickly, volatile carriers dissipate, and residual exposure at re-entry is far below the safety threshold.
| Scenario | Standard REI | Extended REI (sensitive) |
|---|---|---|
| Interior crack-and-crevice | 1 hour | 3–4 hours + ventilation (asthma, pregnancy, infant) |
| Interior surface application (if needed) | 2–4 hours | 4–6 hours |
| Exterior perimeter spray | 2–4 hours | Same (outdoors; dilution is faster) |
| Gel bait only | None (immediate) | None |
| Dust application (outdoor) | Immediate (area safety 24h) | 24 hours for any respiratory condition |
| Heat treatment (bed bugs) | 4–6 hours (thermal) | Same — thermal, not chemical REI |
Ventilation and REI
Ventilation dramatically accelerates the effective REI in practice. The PMRA REI is calculated at worst-case ventilation — a closed, unventilated room. Opening windows to create cross-ventilation (two windows on opposing sides of the home, or a window plus a bathroom exhaust fan) can reduce actual exposure to below the safety threshold in 20–30 minutes for most modern products. Wild Pest technicians recommend cross-ventilation during and after interior treatments. This is good safety practice and also improves treatment efficacy — forced air movement helps the solvent carrier dissipate and allows the active ingredient to bind to substrate faster. For exterior applications, the REI is almost entirely about avoiding wet-surface contact — once the exterior spray dries visibly, exposure risk is minimal regardless of whether the formal REI has elapsed.
When REI matters most
- Households with crawling infants (4–18 months): floor-level contact and hand-to-mouth behaviour increase exposure relative to adults. Observe full REI plus additional ventilation time.
- Asthmatic or COPD households: airway irritant concern from volatile carriers. Observe full REI; schedule treatment when occupant is out; ensure 3+ hours of ventilation after re-entry.
- Pregnancy: precautionary principle applies. Standard REI with ventilation is recommended by PMRA; extending to 3 hours for interior applications is reasonable for peace of mind.
- Caged birds: avian respiratory systems are highly sensitive to airborne compounds. Observe 4–6 hour REI with the bird room fully ventilated before returning the bird.
- Chemical sensitivity (including Long-COVID-related): discuss with Wild Pest on booking. Extended REI, lower-volatility formulation, and away-from-home scheduling are the standard adjustments.