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.

Assumed air changes per hour

Poor1 ACH

Moderate3.5 ACH

Good6.5 ACH

Excellent10 ACH

Voice level factor

Silent×0.35

Conversation×1

Loud×1.8

Singing×2.8

Occupant density factor

Alone×0

Sparse×1

Moderate×1.3

Crowded×1.7

Shoulder-to-shoulder×2.1

The scale accelerates: the step from Crowded to Shoulder-to-shoulder is larger than from Moderate to Crowded, because continuous contact within multiple people's closest-range aerosol zone is a change in kind, not just degree.

Movement factor

Static×0.8

Mixed×1

Circulating×1.35

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.

Mask efficiency (sealed)

No mask0%

SURGICAL45%

KN9582%

N9592%

P10097%

Consistency of wear

None×0

Some×0.3

Most×0.7

All×1

3. Prevalence multiplier

Pulled from the CDC National Wastewater Surveillance System, per pathogen. 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 sites are used; if none report nearby, the statewide picture is used and the result says so.

Airports are treated differently. A flight draws travellers from a whole metro, so instead of averaging, we take the highest county reading within 25 straight-line miles of the airport — separately for each pathogen — and name the county driving it. Counties with no reporting site are skipped rather than counted as low; if nothing in the catchment reports, the airport's own county is used and the result says so.

CDC viral activity level → multiplier

Very Low×0.50

Low×0.75

Moderate×1.00

High×1.50

Very High×2.00

Known limits

Wastewater sites do not cover every county, and coverage within a county varies widely. A single site can represent a small slice of the population.
Data lags reality by roughly one to two weeks, and only COVID-19, influenza A and RSV are tracked.
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.