Questions and answers

For most people, three months of active management brings a new build to a comfortable place, and six months brings it close to a settled baseline. Cabinets continue emitting at declining levels for years, but at levels most people cannot perceive. For someone with MCAS or a significant loss-of-tolerance history, extend those expectations and consider a building finished a year or two ago instead (why that population is different).

In a mild climate with operable windows and acceptable outdoor air, largely yes. Ventilation outperforms filtration for this problem. Filtration earns its place at night, in extreme weather, during wildfire season, and in units where the windows do not open. Buy the ventilation habit first and the equipment second.

No, and this cuts both ways. Formaldehyde odour thresholds vary widely between people, and a significant fraction of the population perceives it only at concentrations near or above the health guideline. Your nose is not a meter. Equally, a strong new-building smell is often solvent VOCs, which resolve quickly and matter less than the formaldehyde you cannot smell (the same logic for musty smells).

It depends on what you will do with the result. Used comparatively — sampling two candidate homes the same way — it can inform a choice. If you would treat a single score as a verdict, the methodological objections apply and you may make a worse decision than you would have without it (what the dispute is actually about). The moisture meter answers a more actionable question for less money.

Cleaning removes settled dust, which is not nothing. It does not address anything inside a wall assembly, and cleaning products themselves add a short-term VOC load. Ask what “professionally cleaned” meant. If it meant a remediation, ask for the documentation — and if you are a tenant, get it in writing.

Often yes in a new build, because circulating air through the filter and through any fresh-air intake beats stagnation. Two caveats: verify the filter is actually rated to do something (the right MERV for the system, and what a filter can and cannot catch), and confirm the system is not pulling from a garage, crawlspace or attic. Continuous fan operation with a poorly located intake distributes the problem rather than solving it.

The often-cited research on plants removing formaldehyde was conducted in sealed laboratory chambers. Translated to a real room with real air exchange, the number of plants required to meaningfully affect concentration is impractical — on the order of many plants per hundred square feet. Keep plants because you like them. Do not count them as mitigation, and note that overwatered plant soil is itself a mold reservoir.

No regulatory body identifies a threshold below which there is zero risk, because formaldehyde is a Group 1 carcinogen and cancer risk assessment does not work that way. What exists are guideline values set to prevent irritation and to keep risk acceptably low — Health Canada's 40 ppb long-term figure and the WHO's 0.1 mg/m³ 30-minute value are the practical anchors used throughout this guide.

Still deciding rather than settling in? The whole thing condenses to four questions, and the beliefs worth dropping first are here.

This article is educational and environmental information. It does not diagnose, treat, or prevent any condition. Work with a qualified clinician on health decisions and with licensed professionals on remediation.

This is the question that decides most cases, and it is asymmetric. An older building carries an unknown history of water that you cannot audit — you are trusting a disclosure and a fresh coat of paint. A new building's main risk is front-loaded, measurable, and fades over months. Known-and-shrinking beats unknown-and-permanent, which is why for most people with a mold sensitivity a newly built home is the better starting point.

If the building is not new, this step is where most of your effort belongs — what you would be taking on is set out in what a water-damaged building exposes you to, and the two profiles are compared directly in water damage vs. new construction.

This is the condition the whole new-build advantage rests on, and it is invisible on a walkthrough. Wet framing closed in behind drywall quietly undoes the entire “no water history” argument. Two things get you most of the way:

• Run the one-hour protocol. A pin-type moisture meter and a hygrometer, under every sink, the patio-door threshold, weather-facing sills, the HVAC condensate pan, the water-heater pan, the base of exterior walls, the bathroom exhaust fan. Under 15% wood moisture is fine; over 19% is stop-and-investigate; indoor humidity 30–50%.
• Ask in writing. Was the framing rained on during construction, and was its moisture verified before the walls were closed? A meter reading is a snapshot; this question covers the months you were not there.

This is the honest cost of the new-build option, and it is a real one. The chemical load is front-loaded and fades — paint in weeks, engineered casework over months (the timeline by material). The question is not whether you mind the idea of it, but whether your particular sensitivity and your particular living situation can absorb a few months of windows open, cabinets open, and a carbon unit in the bedroom.

If the answer is no, that is a legitimate reason to prefer a building that finished off-gassing a decade ago — provided you can satisfy step 1 about its water. That trade is the actual decision, and it is personal rather than technical.

A plan you will not follow is not a plan. Be honest about which of these you will genuinely do: cross-ventilate daily for the first month, keep casework open, keep humidity in band, run a carbon unit where you sleep, re-read the moisture spots seasonally. The month-by-month version is your first 90 days. If the honest answer is that you will do none of it, weight step 3 much more heavily — an unmanaged new build is a worse outcome than a dry old one.

There is no single winner, because the honest answer is “match the machine to the problem.” For spores and dust, buy on HEPA and CADR. For the smell or a new build, buy on the weight and type of carbon, not the CADR headline. Most real homes are fighting both, so the pragmatic pick is one unit that carries true HEPA and a heavy carbon bed, or a HEPA unit paired with a dedicated gas-phase machine. If you want the reasoning behind that split spelled out, read our companion piece on why HEPA-only purifiers keep disappointing people.

Whatever you choose, size it to the room, keep the filters on their replacement schedule, and remember that a purifier manages the air — it never replaces fixing the moisture source that let mold grow.

• Sampling variability — dust is patchy. Where and how you sample can move the score, so critics argue a single number carries more certainty than the method supports.
• Scope creep — ERMI was built to compare populations in research, not to grade one house; using it as a pass/fail for a specific home is using it outside its validated purpose (the EPA’s own point).
• Clinical vs environmental framing — HERTSMI-2 is a tool from a specific clinical school of thought; how much weight to give it depends on whether you accept that framework, which is itself contested.

• Mold spores, dust, and allergens after remediation → HEPA (particles).
• A musty mVOC odor → a large amount of standard activated carbon.
• New-construction formaldehyde → impregnated carbon rated for formaldehyde, not HEPA.
• Most real situations → both, in enough quantity: a true-HEPA-plus-heavy-carbon unit, or a HEPA unit paired with a dedicated gas-phase unit.