What we know, what we don't, and why it's wastewater
Every household in the Bay Area contributes to the same data set, twice a day. Treatment plants sample it, public health labs measure the viruses in it, and Sniffcast turns those numbers into a forecast you can read before school drop-off.
Where the numbers come from
The heart of Sniffcast is Bay Area wastewater, because it's local and it moves first. Around it we put what doctors, labs and hospitals report, so you can see whether the clinics agree. Each source is labeled with where it covers: Bay Area, California, or our CDC region.
- CDPH Cal-SuWers wastewater surveillance (Bay Area): COVID-19, flu A and B, RSV and norovirus at about 30 Bay Area treatment plants. Samples are run by CDPH's own lab, WastewaterSCAN (Stanford and Emory), SCAN, and CDC contract labs, and CDPH publishes them all as California open data.
- CDC National Wastewater Surveillance System (Bay Area and California): measles, H5 bird flu and mpox, shown as detected or not detected.
- CDC National Syndromic Surveillance Program (California): the share of ER visits for flu, COVID-19, RSV and any respiratory illness.
- CDC NREVSS lab testing (California, Arizona, Nevada and Hawaii together): the percent of clinical lab tests that come back positive. This is how we see plain colds (rhinovirus and enterovirus), croup viruses (parainfluenza), adenovirus, seasonal coronaviruses and HMPV, which wastewater doesn't track.
- CDC NHSN hospital data (California): children newly admitted to the hospital with flu, RSV or COVID-19.
- CDC New Vaccine Surveillance Network (seven U.S. children's hospitals): EV-D68 among sick school-age kids. It's national and a few weeks behind, so we use it as a heads-up only.
- CDC ILINet via CMU Delphi (California): the share of doctor visits for flu-like illness.
- EPA AirNow: current air quality by county, when connected.
- The calendar: holidays and school-year seasons that tend to move germs around. We only name exact dates when they're fixed. School breaks vary by district.
The forecast updates every morning. Labs and agencies need a few days to a couple of weeks to publish, so "today" means "the latest numbers each source has released." The home page lists every source with its latest date and whether it's working.
Every number is a link
Each number in the forecast is tied to the data it came from. Hover over it (or tap it on a phone) to see a small chart with the two values being compared and their dates. Click it to jump to the full chart or card. The daily paragraph can't contain a number that isn't one of these links: our code checks every one before it's published.
What the levels mean
Virus concentrations can't be compared between treatment plants. They serve different numbers of people, with different pipes and different rain. So each plant is compared with itself: today's level against every day that plant measured in the past year. The Bay Area figure is the median across reporting plants.
| Level | Today's level is higher than… | In plain terms |
|---|---|---|
| Probably Fine | less than 33% of the past year | Low |
| Probably Fine-ish | 33% to 65% | Moderate |
| Less Fine | 66% to 89% | High |
| Not Fine | 90% or more | Very high |
Lab-test and clinic numbers use the same scale: this week's value against the past year of that same signal. The forecast card at the top shows the highest level among the viruses in Bay Area wastewater, with the number behind it, because that's the local signal. Trends compare a smoothed 10-day average with the same average a week earlier. "Creeping up" means up at least 15%, and "climbing fast" means up 50% or more.
"What's likely to come home" ranks viruses by three things: their level, which way they're heading, and how often each one is what sends school-age kids home. It's an informed guess, not a probability.
What wastewater can't tell you
- How many people are sick. It measures how much virus is around, not cases. Some people shed a lot, some barely at all.
- What's in your kid's classroom. A treatment plant serves tens or hundreds of thousands of people. Your school can have a stomach bug week while the region is quiet, and the other way around.
- Everything that goes around. Wastewater can't see strep, ear infections, plain colds, or hand, foot and mouth. Lab testing fills in colds, croup viruses and adenovirus, but only for the whole region, not your town. Strep and ear infections we still can't see.
- A diagnosis. The symptom decoder is a hint based on what's circulating. Your pediatrician and a home test beat us every time.
Who writes the daily forecast?
The numbers are computed by open code. The daily paragraph is written by an AI model (Claude) from those numbers, or by a plain template when the model isn't available. The model only sees the computed facts, and it can't type numbers at all: it writes a reference to a fact, and the site fills in the value. If it writes a digit anywhere else, or refers to a fact that doesn't exist, its paragraph is thrown out and the template is used instead. If something reads wrong, tell us.
Sharing data with Sniffcast
Run a county health department, a pediatric practice, a school district nurse's office, a testing lab or a wastewater program? If you can share a weekly count or percentage for the Bay Area, Sniffcast can show it beside the public data, credited to you and labeled with exactly what it covers. A simple CSV with a date, a measure and a value is enough. We never publish anything that could identify a person.
Not medical advice
Sniffcast is not medical advice and is not a substitute for your child's doctor. The "what helps" and "call the doctor" notes are general guidance drawn from the CDC, CDPH and the American Academy of Pediatrics. If your child has trouble breathing, signs of dehydration, a fever lasting more than three days, or you're worried, call your doctor. In an emergency, call 911.