Radon Risk in Boston, MA and the Greater Metro Area: What Renters and Owners Should Know

Boston has some of the oldest housing stock in the country. That history is part of the city’s charm—but it also creates an easy-to-miss gap with modern indoor-air issues like radon, especially in older rentals and multi-family buildings across Greater Boston.

In many triple-deckers, brownstones, and pre-war apartment buildings—from Cambridge and Somerville to Brookline, Medford, Newton, Quincy, and Waltham—basements and lower levels aren’t “unused space.” They’re where people do laundry, store strollers and bikes, swap seasonal bins, check the boiler, or carve out a small home gym or work area. Garden-level apartments are common, too, and they’re often marketed as convenient and affordable without much discussion of what “closest to the ground” can mean for radon testing.

That’s the central knowledge gap in radon risk Boston MA: not that every building is high, but that older foundations and rental turnover can keep radon off the checklist. People assume “radon is a suburban problem,” or that an older, draftier building can’t have radon. In reality, results can differ dramatically from one building to the next—even among similar-looking homes on the same block in Somerville or a quiet side street in Brookline. The practical approach stays consistent: know when to test, test correctly, and mitigate when results call for it.

1. Radon risk across Greater Boston

Radon is a naturally occurring radioactive gas that forms in soil and rock and can enter buildings through openings and gaps in the foundation. You can’t see or smell it, which is why it often gets overlooked during a renovation, a unit turnover, or even a routine safety conversation.

The main concern is long-term exposure. The EPA says any radon carries some risk, and the EPA “action level” is 4.0 pCi/L—meaning mitigation is generally recommended at or above that level. Many health organizations also encourage considering mitigation for results between 2.0 and 4.0 pCi/L, especially when people spend a lot of time on lower levels.

The most useful local takeaway for Greater Boston isn’t a blanket label like “Boston is high” or “Boston is low.” It’s that radon can be highly variable in the metro area:

• Differences can show up neighborhood-to-neighborhood due to underlying geology and urban fill.
• Differences can show up building-to-building because foundations, air sealing, ventilation, and pressure dynamics vary—especially in older properties.
• Differences can show up unit-to-unit in multi-family housing, particularly between a garden-level unit and higher floors.

So yes—Cambridge, Somerville, Brookline, Quincy, Medford, Newton, and Waltham can all include homes that test low and others nearby that test higher. In practice, “local radon knowledge” comes from testing the specific building (and the right level), not assumptions based on zip code.

For local resources and guidance across communities, you can also browse: Boston and Massachusetts Radon Mitigation.

2. Older foundations and basements

Older Boston-area buildings tend to have features that make radon pathways harder to predict and easier to ignore—especially when a basement is treated as a utility zone rather than living space.

Common conditions in older housing stock include:

• Stone, brick, or rubble foundations with many joints and penetrations
• Fieldstone basements that are difficult to fully seal
• Settling and hairline cracks in slabs and walls
• Unsealed utility penetrations (pipes, conduits, sump pits)
• Older floor drains or gaps along perimeter edges
• Renovations that change air pressure, like air sealing, new windows, updated HVAC, or finishing a basement

Where the Greater Boston angle really matters is how these basements are used day to day. In a Newton single-family or a Waltham two-family, the basement might be “just storage,” but it’s still the place people return to regularly for laundry, tools, holiday bins, or a quick workout. In Cambridge, Somerville, and Brookline, it’s common for residents to pass through basement entries, use bike rooms, or spend time in garden units that sit partly below grade.

Radon risk is often highest on the lowest occupied level because that’s where soil gas entry is most direct. In multi-family buildings—including triple-deckers and older apartment houses—this is why a garden unit can have a different radon profile than a third-floor unit, even when they share the same address and the same landlord.

3. Radon Levels in the Greater Boston Area and Surrounding Communities

RadonResources.com user-submitted radon tests in the Boston, MA area show a pattern residents should take seriously: across 15 submissions from the metro and nearby communities, the average reported level is 5.9 pCi/L (median 5.0 pCi/L). Importantly, 80% of submissions are at or above 4.0 pCi/L (the EPA action guideline), and 13% are 10 pCi/L or higher. The highest submitted reading in this dataset for the metro is 19 pCi/L. Because radon can vary dramatically from one house to the next—even on the same street—the takeaway isn’t to guess; it’s to test your specific home, especially if you have a basement, garden unit, crawlspace, or spend a lot of time on the lowest level.

City	Submissions	Avg pCi/L	Median pCi/L	% ≥ 4.0	Max
Medford, MA	3	8.0	5.0	100%	15
Somerville, MA	3	4.3	5.0	67%	6
Quincy, MA	2	10.5	10.5	50%	19
Malden, MA	2	5.0	5.0	100%	5
Arlington, MA	2	4.0	4.0	50%	5
Cambridge, MA	1	5.0	5.0	100%	5
Everett, MA	1	4.0	4.0	100%	4
Waltham, MA	1	4.0	4.0	100%	4

If you’re searching for “Greater Boston radon levels,” it helps to treat any map or regional summary as a starting point, not a prediction. Older buildings and rentals add another layer of uncertainty: two similar properties—say, one in Medford and one in Quincy—can behave differently based on foundation condition, air sealing, and how lower levels are used.

Why variability is especially relevant in the Boston metro:

• Geology and soil can change over short distances.
• Human-made fill (common around older urban development) can affect how soil gas moves.
• Foundation type and condition differ widely across older housing stock—from brownstones to pre-war apartment buildings.
• Seasonal living patterns matter: Boston winters mean closed windows and stronger “stack effect,” which can pull more soil gas into lower levels.
• Building pressure changes from exhaust fans, dryers, range hoods, and HVAC can influence radon entry, including in shared basements.

If you’re in Cambridge, Somerville, Brookline, Quincy, Medford, Newton, or Waltham, the best “local” guidance is still straightforward: assume radon is possible, and make decisions based on testing—especially when a garden/basement space is used for living, working, or sleeping.

For more on how to set up a test and what it measures, see: Radon Testing.

4. Radon testing in rental properties

Radon in rentals is where Greater Boston’s older housing stock tends to create the biggest knowledge gap. Renters often assume testing is routine. Owners may associate radon with single-family suburban homes. Property managers may focus on visible life-safety items—smoke alarms, CO detectors, egress—while radon stays invisible and therefore easy to postpone.

In Boston-area rentals, the details matter: a “basement test” in a shared laundry/mechanical area may not represent a garden apartment where someone sleeps. Likewise, a test on an upper floor won’t tell you what’s happening in a basement office space that tenants use daily.

What renters should know

• Radon is measured with a test device placed in the lowest livable level (or lowest level that could reasonably be occupied).
• A radon result is about the building, not the tenant—so requesting prior results is reasonable during leasing or renewal, especially for garden-level units in places like Cambridge, Somerville, or Brookline where those layouts are common.
• Short-term tests are useful for screening, but long-term tests give a better picture of average exposure.

Short-term vs. long-term tests (plain language)

• Short-term test: typically 2–7 days (some are 48 hours). Good for a quick check. Results can swing due to weather and how the building is operated.
• Long-term test: typically 90+ days. Better at estimating the true annual average.

In practice, a simple Boston-metro-friendly approach is:

• Use a short-term test for quick screening (for move-in decisions, concerns about a garden unit, or a finished basement used as an office).
• Follow up with a long-term test when results are borderline or when you want a more stable picture—useful in older buildings where airflow changes with season.
• If results are at or above 4.0 pCi/L, shift the conversation toward mitigation rather than repeating short tests indefinitely.

Renter-focused question list (copy/paste) If you’re renting (or renewing) in Boston or nearby—Newton, Waltham, Medford, Quincy, Cambridge, Somerville, or Brookline—especially in a garden-level unit, ask:

1. Has this unit or building ever been tested for radon? If yes, can I see the date and results?
2. Where was the test placed? (Basement vs. garden unit vs. first floor matters.)
3. Was it a short-term or long-term test?
4. If mitigation was installed, what system is it and when was it serviced last?
5. Are there any finished basement areas, bedrooms, or offices below grade?
6. Are laundry/storage areas in the basement frequently used by residents?
7. Who pays for testing or mitigation if elevated levels are found? (Put the plan in writing when possible.)

Because Massachusetts requirements for rental radon testing/disclosure can vary over time and by situation, it’s smart to confirm current expectations with Massachusetts DPH and/or your local health department rather than relying on hearsay.

5. Owner responsibilities

Owners and property managers don’t need to be radon specialists to manage radon responsibly. In Greater Boston, the most effective approach is consistent process—especially in older multi-family properties where tenants change, basements get repurposed, and “common areas” still count as lived-in space for practical exposure.

A landlord in Somerville managing a triple-decker, or a property manager overseeing older apartments in Brookline or Cambridge, may have multiple lower-level spaces in play: laundry rooms, storage cages, bike rooms, and sometimes a garden unit with regular occupancy. The more the basement is part of everyday life, the more important it is to avoid the “we never talked about radon” pattern.

Owner/landlord action list (practical and defensible)

• Decide a testing policy (for example: test garden/basement units and common-area basements on a schedule, and after major envelope/HVAC work).
• Keep records: test dates, device type, placement location, conditions, results, mitigation documentation, and post-mitigation retest results.
• Plan for turnover: radon questions often come up at leasing time—have a standard answer and documents ready.
• Retest after changes: sealing, finishing basements, HVAC changes, adding exhaust fans, or major foundation work can change radon behavior.
• Communicate clearly with tenants: what you tested, what it means, and what you’ll do if results are elevated.

Just as important: avoid overpromising legal conclusions. Massachusetts radon rules and expectations for rentals can change and may differ based on building type, program requirements, financing, or municipal guidance. Check current information through MA DPH and local health departments, and consider professional advice when setting policy.

6. Interpreting results

Radon results are reported in pCi/L (picocuries per liter).

Key interpretation points:

• Any radon carries some risk. Lower is better.
• The EPA action level is 4.0 pCi/L: at or above this level, mitigation is generally recommended.
• Results can vary with seasons, weather, and building operation (windows closed, heating on, stack effect).

A simple way to think about common outcomes

• Below 2.0 pCi/L: Often considered relatively low. Keep records and consider retesting later or after renovations.
• 2.0 to 3.9 pCi/L: Not an emergency, but worth taking seriously—consider a long-term test or discuss mitigation depending on occupancy and risk tolerance.
• 4.0 pCi/L or higher: This is the EPA action level—plan next steps for mitigation and confirm with follow-up testing after work is completed.

Placement and “lowest level” matters in Boston rentals In an older multi-family building, “the lowest level” can be more than one space: a shared basement laundry room, a semi-finished storage/work area, and a separate garden apartment entrance. A test in a shared basement doesn’t automatically represent a garden-level unit—and a first-floor test doesn’t represent a below-grade bedroom or office.

If a space is occupied (or used regularly), test that level. This comes up frequently in Cambridge and Somerville triple-deckers, Brookline brownstones, and mixed-use buildings in Quincy or Waltham where layouts have evolved over decades.

7. Mitigation basics

If elevated results come back, mitigation is usually straightforward and common across the U.S., including radon mitigation Massachusetts projects in older housing.

What mitigation typically looks like The most common system is active soil depressurization (ASD):

• A pipe is routed from beneath the slab (or from a sump area) to above the roofline.
• A quiet inline fan pulls radon-containing soil gas from under the building and vents it safely outdoors.
• Cracks and openings may be sealed as supportive work (but sealing alone is rarely enough).

What to expect in older Boston-area buildings

• Complex foundations (stone/rubble) can require a more customized design.
• Multiple slabs or sections (common in older multi-family basements) may need careful diagnostics.
• Finished basements or tight mechanical rooms can affect routing and installation time.
• Garden-level units may need special attention so the system addresses the spaces where people actually spend time—sleeping, working, or doing laundry.

After mitigation: always retest A mitigation system should be followed by a post-mitigation test to confirm results dropped. Keep that documentation for tenants, buyers, and future property managers.

8. Clear next steps checklist

Use this as a simple plan whether you’re a renter, owner, or homeowner in Boston, Cambridge, Somerville, Brookline, Quincy, Medford, Newton, or Waltham.

Today (15–30 minutes)

• Identify the lowest occupied level (or any garden/basement areas used regularly for living, working, or sleeping—even if it’s “just laundry and storage” most weeks).
• If you’re renting, send a written question to the landlord/property manager requesting:
  • Any prior radon test results and dates
  • Whether any mitigation system exists and maintenance history
• Save links for local reference: Boston and Massachusetts Radon Mitigation.

This week

• Choose a testing approach:
  • Short-term test if you need a quick screening (move-in decisions, immediate concerns).
  • Long-term test if you want a better estimate of average exposure.
• Ensure correct placement:
  • Lowest livable level, away from drafts, exterior doors, and direct heat sources.
• Keep basic conditions consistent during short-term testing (closed-house conditions if applicable).

This month

• Review results using the EPA action level 4.0 pCi/L as your decision point.
• If results are ≥ 4.0 pCi/L:
  • Get mitigation quotes and confirm the plan includes post-mitigation testing.
  • For multi-family buildings, clarify whether mitigation addresses the relevant unit(s) and shared lower areas.
• If results are 2.0–3.9 pCi/L:
  • Consider a long-term test, especially if a basement/garden area is occupied.
  • Discuss practical mitigation options if the space is frequently used (office, bedroom, workout area, play area, or frequent trips for laundry/storage).
• Retest after major renovations, HVAC changes, or basement finishing.

FAQ

1. Is radon mainly a problem in suburbs, not Boston? No. Radon can show up in urban areas too, and in Greater Boston it can vary building-to-building. Older basements, garden-level units, and foundation openings make radon relevant in city and inner-suburb housing—from Cambridge and Somerville to Quincy and Medford.

1. If I live on an upper floor, do I need to worry? Risk is usually highest on the lowest occupied level, but building airflow can move radon. If you never use lower levels, your exposure may be lower—but it’s still worth knowing if the building has a garden unit, a basement office/bedroom, or frequently used laundry and storage areas.

1. What’s the difference between short-term and long-term radon tests? Short-term tests (2–7 days) are quick screens. Long-term tests (90+ days) better estimate your average exposure across changing weather and seasons.

1. What level is considered “too high”? The EPA action level is 4.0 pCi/L—mitigation is generally recommended at or above that number. Any radon carries some risk, so lower is better.

1. Are landlords in Massachusetts required to test or disclose radon? Requirements and practices can vary and may change over time. Check current guidance from MA DPH and your local health department, and get any radon-related agreements in writing.

Conclusion: close the radon knowledge gap—one good question at a time

The biggest issue with radon risk Boston MA isn’t panic—it’s the quiet gap created by older housing stock and rental routines. In triple-deckers, brownstones, and pre-war apartment buildings across Greater Boston, radon can go untested simply because the basement is treated like “background space” and the next lease signing arrives before anyone asks.

If you’re renting—whether in a Cambridge garden unit, a Somerville triple-decker, a Brookline brownstone, or an older building in Quincy, Medford, Newton, or Waltham—ask for prior results, testing location details, and any mitigation documentation. If you’re an owner or property manager, set a clear testing policy that reflects how people actually use lower levels (laundry, storage, bike rooms, and garden apartments), and be ready to mitigate when results meet or exceed 4.0 pCi/L.

To keep the process simple and local, start with your community resources here: Boston and Massachusetts Radon Mitigation, then follow the step-by-step testing guidance at Radon Testing.