Why August is Chicago's worst month for restaurant inspections.

We bucketed 20,170 Chicago food inspections by the month they happened. The fail rate isn't flat.

December: 14.3% fail rate. August: 21.6%. A 7.3-percentage-point gap. In relative terms, an August inspection is 51% more likely to fail than a December one.

Chicago does several types of inspections — routine canvass visits, follow-up reinspections, complaint responses, license checks. Different types fail at different rates. So an obvious question is: maybe December's inspection mix shifts toward easier-to-pass types (like reinspections of places already in good shape).

We tested it. Even comparing apples to apples — looking only at routine Canvass inspections — December still fails far less often than August.

Pest evidence — rodent droppings, live roaches, fly activity — is one of the largest categories of critical violation. And pest populations in Chicago are extremely seasonal. Roaches and flies need warmth to reproduce. Rodents seek shelter year-round but their visible activity peaks in summer when food sources are abundant.

When a Chicago kitchen gets inspected in August, the inspector is more likely to see something — droppings, a live roach, gnaw marks, fly activity around dumpsters. By December, the same kitchen looks dramatically cleaner because the pests are dormant, not because the kitchen has changed.

This is partial. Restaurants also prepare more carefully for the holiday rush in December — better staffing, busier service, more attention. But the magnitude of the seasonal swing is too large for behavior alone. The pest cycle is doing most of the work.

A clean December record tells you less than a clean August record. The same kitchen, inspected in different seasons, can look like two different operations.

• If a restaurant's last inspection was in winter, look at its summer history. A clean cold-weather inspection doesn't tell you how the kitchen handles July humidity and August pest pressure.
• Multiple inspections matter more than one. A restaurant with a year of clean records (across all seasons) is meaningfully different from one with only winter clean records.
• Pest-related citations specifically matter more in their season. A roach citation in March is unusual and probably real. A roach citation in August is common but still actionable.
• If you're an owner: your summer prep determines your annual fail rate. Sealing entry points, scheduling pest control, and tightening dumpster routines in late spring is when most of the year's inspection risk gets decided.