Methodology
How we build and check this site
Three state license lists are the spine. Everything else on a listing comes from the business's own website or its Google profile, with the date we read it.
Sources and how often we read them
| Source | Publisher | How | Read again | Last read |
|---|---|---|---|---|
| IEMA-OHS Radon Active Licensee Listing | IEMA-OHS | The public list page, read through its own paging links (no search form submitted) | Monthly | October 15, 2026 |
| DEP Radon Services Directory | PA DEP | The monthly PDFs of certified mitigation and testing firms | Monthly, after DEP posts the new edition | October 1, 2026 |
| Professional and Occupational Licenses in Colorado (DORA, open data) | Colorado DORA | The open data API, radon mitigation (RMT) and measurement (RME) license types | Monthly | October 15, 2026 |
| Home inspector licenses: IDFPR, CT DCP, DE DPR | State open data portals | Active rows of each file; counts only | Monthly | October 15, 2026 |
| EPA map of radon zones (county spreadsheet) | US EPA | County zone table for IL, PA and CO | When EPA posts a new file | October 15, 2026 |
| Business websites | Each listed business | Home page and up to four of its own pages, one request a second | New businesses monthly; every site again each quarter | October 15, 2026 |
| Google Business Profiles | Google, via a maps data service | Matched by the phone number the state list prints | New businesses monthly; every profile again each quarter | October 15, 2026 |
We read only pages a site's owner allows automated readers to visit. A site that asks us not to is skipped, and no copy of its files from elsewhere is used.
When a radon business gets a listing page
- It is on the Illinois IEMA-OHS list or the Pennsylvania DEP directory for mitigation, and is based in that state.
- Its Google profile carries the phone number the state list prints, is open, and its name agrees with the list's. A person named on the list is never matched by name.
- It carries a fact from its own side: radon work it names in its own words on its own website, or a price it publishes for its own work. A second license on the same list is shown but does not count on its own.
- The page shows at least three sourced facts in all, checked on the built page before every deploy.
Today 48 businesses pass: Illinois 17 of 73, Pennsylvania 31 of 78. The rest stay in the state tables with their license numbers. Why they are held:
| Reason held | Illinois | Pennsylvania |
|---|---|---|
| no Google profile with the list's phone | 36 | 32 |
| no first-party fact yet | 16 | 14 |
| Google profile is an association or directory | 2 | 0 |
| Google profile name differs from the list's business name | 2 | 1 |
| second office of a listed business | 1 | 0 |
Colorado's file names people, not companies, so its licenses appear in a table and no Colorado business has a page yet. Businesses licensed only to test, and home inspectors, are counted and not listed one by one yet.
Prices
A price appears only as a licensed business prints it for its own work, on its own website, with a verbatim quote we matched on that page and the date we read it. Nothing is averaged. A range for a kind of work needs five or more businesses; today 7 publish a standard system price. Each price is read again every month and leaves the site when it leaves its page.
Rules and figures
Each rule is a short quote from the statute, rule text or agency page, rewritten in plain words beside its link and read date. Before a deploy, every quote is matched against its page again; a quote that no longer matches is removed or corrected and logged on the corrections page.
How the site grows
New listing pages are added in small batches, only after the pages already live have been read by search engines for several weeks. A business held today can be listed later, once it passes the rule above.