Methodology
The score is deliberately simple and fully open. It is a relative index, not an infection probability. Two numbers are multiplied:
final score = base exposure score × local prevalence multiplier
1. Base exposure score (0–10)
A property of the activity alone: how much air you share, with how many people, for how long, and how well that air is replaced. Indoors starts from a base of 6.5 and outdoors from 1.4 with a hard ceiling of 2.5, because open air disperses aerosols faster than any indoor room. Duration uses a saturating curve (exponent 0.45), so four hours is about 1.4× the dose of two hours, not double.
2. Masks
Protection is the respirator's sealed efficiency multiplied by how consistently it is worn. A great mask worn half the time is not a great mask.
3. Sports venues
A stadium does not reduce to indoor or outdoor. A fully open-air seating bowl gets the same hard outdoor ceiling as any other outdoor scenario. A roof overhead with open sides — including a retractable roof left open — gets an intermediate ceiling 40% above the open-air one, since the roof traps some aerosol but the sides still dilute it. A fully enclosed arena, or a retractable roof closed for the event, uses full indoor scoring.
Concourses, bathrooms and beer lines are always scored as enclosed, poorly ventilated and one density tier tighter than your seats, whatever the bowl looks like. The seat and circulation sub-scores are blended by how long you spend in each, with a small uplift so a short, very bad concourse stretch still registers.
4. Prevalence multiplier
Pulled from official wastewater surveillance, per pathogen: the CDC National Wastewater Surveillance System for US locations, and the Public Health Agency of Canada (PHAC) wastewater monitoring dashboard for Canadian locations. The result always names which source and which specific location it drew from.
US locations: your location resolves to a county plus its directly adjacent counties; every reporting site in that band is averaged, weighted by the population it serves. If no site reports in your county, neighbouring counties are used; if none report nearby, the statewide picture is used and the result says so.
Canadian locations: Canada's wastewater network has far fewer reporting sites — about 40 nationally — so instead of averaging nearby counties, we search an expanding radius outward from your location until an active, baselined site is found, and use that reading directly. The result names the site and how far it is from you. Sites without at least 12 months of data, or without recent submissions, are treated as inactive and skipped.
Airports and sports venues (both countries): a flight or a big game draws people from a whole region, so instead of averaging, we take the highest reading within 25 straight-line miles of the venue — separately for each pathogen — and name the specific location driving it. For US venues this searches counties; for Canadian venues this searches the same expanding radius used for Canadian locations generally. Locations with no reporting site are skipped rather than counted as low; if nothing in the catchment reports, the fallback described above for that country is used and the result says so.
Cross-border flights: for a flight between a US and Canadian airport, each leg's prevalence is pulled from its own country's data source, per pathogen. Since Canada reports influenza A and B separately while the US reports combined influenza A only, flu comparisons across a cross-border flight use influenza A on both sides; influenza B is shown for the Canadian leg alone where available, without a US-side equivalent to compare against.
Known limits
- Wastewater site coverage varies significantly by location and by country. US coverage is denser and county-based; Canadian coverage is sparser (roughly 40 sites nationally) and location-based via expanding radius search. A single site can represent a small slice of the population it's meant to reflect.
- Data lags reality by roughly one to two weeks in the US, and can lag somewhat longer for Canadian sites depending on sample transport and lab processing time. COVID-19, influenza A and RSV are tracked for both countries; influenza B is additionally tracked for Canadian locations only.
- The multipliers are calibrated judgement built on published aerosol-transmission research, not measured infection rates. Compare scenarios to each other rather than reading a score as an absolute chance of infection.
- Individual factors — vaccination, immunity, age, medications — are not modelled.