CleanZone Method
The Threshold Table: Setting Your Own Non-Negotiables
Most relocation research starts backwards: open a map, scroll around, feel a vague dread or relief about a neighbourhood, repeat. That process never terminates, because "feels fine" has no stopping rule. The fix isn't more data. It's deciding, in advance and in writing, exactly what number ends the conversation for each factor — before you look at a single postcode.
At a glance
- A threshold table is a personal pass/fail line for every factor you care about, written down before you start comparing places — not while you're standing in one, already half in love with the kitchen.
- Every number in your table should trace back to a published standard — WHO, EPA, EU Euratom, USGS, FEMA — not a gut feeling or a forum post.
- Hard gates eliminate a candidate outright if crossed. Soft preferences get weighted into a score but never disqualify alone.
- Mixing the two — treating every preference as a gate, or every gate as negotiable — is why relocation research drags on for months without a decision.
- Build the table once, reuse it for every candidate; a ready-made version lives at the printable checklist.
The problem a threshold table solves
Ask someone why they crossed a neighbourhood off their list and you'll usually get a story, not a number: "it felt a bit close to the flight path," "the water tasted off," "there was a lot of traffic at night." Every one of those is a real signal poorly measured. The story format is what keeps the search open-ended — a story can always be argued with, softened, revisited after a nice viewing. A number written down last month, before you saw the house, cannot.
That's the actual function of a threshold table: it moves the emotionally loaded decisions to a moment when you're calm and not standing in anyone's kitchen, and it converts every later comparison into arithmetic. You're not deciding whether this radon reading is "acceptable" while the agent is waiting outside. You decided that in advance. Now you're just checking whether 410 Bq/m³ is above or below the 300 you wrote down three weeks ago.
Anxiety about relocation risk is usually anxiety about an undefined stopping rule, not about the risk itself. Once every factor has a number and a public source, the anxiety has nowhere left to attach — you're either above the line or you're not.
Hard gates vs soft preferences
The single most common threshold-table mistake is treating every row the same way. It produces two failure modes: a table so strict it eliminates every real place on earth, or a table so loose it never eliminates anything and stops being useful. The fix is a deliberate split.
A hard gate is binary. Cross it, and the candidate is out, full stop, no matter how good everything else looks. Reserve hard gates for factors with a real, published, hazard-linked threshold and a real consequence for ignoring it — the kind of thing a regulator set a legal or health-based limit for. Radon above 300 Bq/m³ (EU Euratom Directive 2013/59) or 4 pCi/L (EPA action level, ≈148 Bq/m³) is a hard gate: mitigation is knowable and the health basis is not in dispute, but a table that "prefers" lower radon rather than requiring it isn't doing its job. A property inside a 1% annual-exceedance-probability floodplain (the FEMA/EU standard definition of a "100-year floodplain") is a hard gate for the same reason — insurance, resale, and safety all hinge on that exact line.
A soft preference is everything with a real published reference value but no comparable binary stakes — water hardness, aircraft overflight frequency, EMF near infrastructure operating well inside ICNIRP reference levels. These get a score, contribute to ranking, and lose to a genuinely great property on every other axis. Turning a soft preference into a hard gate is how people talk themselves out of good, affordable places over a factor that was never going to affect them.
Anchoring your numbers to a public standard
The table only works if every threshold is defensible — meaning you could show it to a sceptical friend, a surveyor, or your future self at 2am and it would hold up. That rules out round numbers you invented and vague terms like "not too noisy." Below are eight factors people commonly threshold, each with the actual published reference value and the body that set it, plus whether it's typically a hard gate or a soft preference.
| Factor | Public standard | Source | Typical treatment |
|---|---|---|---|
| PM2.5 (fine particulates) | 5 µg/m³ annual mean | WHO Global Air Quality Guidelines, 2021 | Hard gate for respiratory-sensitive households |
| Night noise | 40 dB L_night | WHO Environmental Noise Guidelines for the European Region | Soft preference — hard gate if sleep-disordered |
| Indoor radon | 300 Bq/m³ (EU) / 4 pCi/L (US) | EU Directive 2013/59/Euratom; US EPA action level | Hard gate, confirmed by a long-term test |
| Water hardness | >180 mg/L CaCO₃ = "very hard" | USGS water-hardness classification | Soft preference (appliance/plumbing cost, not health) |
| Flood risk | 1% annual exceedance probability | FEMA Special Flood Hazard Area / EU Floods Directive | Hard gate — insurability and resale both hinge on it |
| Aircraft overflight noise | 45 dB L_den (WHO 2018); EU END maps from 55 | WHO Environmental Noise Guidelines 2018; Directive 2002/49/EC | Soft preference — weighted with frequency, not just level |
| RF/EMF near infrastructure | General-public reference levels (frequency-dependent) | ICNIRP 2020 Guidelines | Soft preference — precautionary, not a health hard gate |
| Soil / groundwater contamination | Site-specific Regional Screening Levels | US EPA RSLs; EU national contaminated-land registers | Hard gate on any confirmed exceedance at a known site |
Notice the ICNIRP and EU noise-directive rows don't have a health-based "danger line" the way radon and PM2.5 do — they're precautionary or annoyance-based reference values. That's not a reason to ignore them; it's a reason to keep them as soft preferences instead of promoting them to gates they were never designed to be.
A gauge reads one value against the line
Once a row has a threshold, checking a candidate against it is a single comparison — the value either sits inside the acceptable band or spikes past it. That's worth visualising the same way every time, so a family scanning ten candidates in an evening can pattern-match the shape instead of re-reading eight rows of text per house.
The worked table
Here's a threshold table filled in the way you'd actually use one — one row per factor, your own hard or soft treatment, the public anchor, and your personal number. Copy the structure, replace the numbers with ones that reflect your household's actual sensitivities (a household with asthma sets PM2.5 tighter than WHO's guideline; a household without a basement can drop radon's weight, though never its gate status if a lowest-occupied-floor test is planned).
| Factor | Type | My threshold | Anchored to |
|---|---|---|---|
| PM2.5 annual mean | Hard gate | ≤ 8 µg/m³ | WHO AQG 5 µg/m³ + margin |
| Night noise | Soft preference | prefer ≤ 40 dB, penalise above | WHO L_night 40 dB |
| Indoor radon (post-test) | Hard gate | ≤ 200 Bq/m³ | EU 300 / EPA 148 Bq/m³, tightened |
| Flood zone | Hard gate | outside 1% AEP zone | FEMA / EU Floods Directive |
| Water hardness | Soft preference | prefer ≤ 120 mg/L | USGS "moderately hard" ceiling |
| Aircraft overflight | Soft preference | prefer ≤ 50 dB L_den | EU END 55 dB mapping trigger, tightened toward WHO 45 |
A threshold table with twelve hard gates is not a stricter search — it's a search that returns zero results, because no real property clears every regulator's guideline value simultaneously. Reserve hard-gate status for the handful of factors where crossing the line has a genuine, named consequence for your household. Everything else belongs in the weighted column.
Using the table against real candidates
Once the table exists, evaluating a candidate location is mechanical: pull the relevant public figures or grid fields for the address — pm25, noise, radon, hardness, flood zone, flights — and run each one against your row. Any hard gate crossed removes the candidate immediately, no further comparison needed. Everything that survives the gates gets scored on the soft preferences, and that score is what you actually compare between the two or three places left standing. This is the same logic behind CleanZone's own the printable checklist — a ready-made version of this exercise you can fill in per candidate rather than rebuild from scratch each time.
Without a table
Every viewing reopens the whole decision. A nice kitchen quietly outweighs a radon reading nobody actually tested. "Feels safe" substitutes for a number. The search has no defined end, so it runs for a year.
With a table
Gates are checked first and are non-negotiable, in five minutes, before a viewing is even booked. Preferences are scored, not argued about. The search ends when a candidate clears every gate and scores highest on the rest.
Keeping it honest
Two disciplines keep a threshold table from quietly rotting. First, revisit it only when your household's actual circumstances change — a new asthma diagnosis, a planned basement conversion, a pregnancy — not because one property you liked happened to fail a gate. Moving the goalposts to fit a house you've fallen for defeats the entire purpose. Second, write the source next to every number, not just the number. "300 Bq/m³ (EU Euratom)" survives a memory lapse eight months later; "300" alone does not, and you'll find yourself unable to defend — or even recall — where it came from.
A threshold table doesn't make a place safer. It makes the decision about that place fast, defensible, and yours — built once from public standards (pm25, noise, radon, hardness, flood zone), then reused against every candidate. Start one at the printable checklist.