## Introduction

The International Agency for Research on Cancer defines radon as a category 1 carcinogen. This is because 222Rn emits alpha particle ionizing radiation that damages DNA in a way nearly impossible to heal without genetic errors that drive cancer formation14,15,16. Radiation susceptibility in terms of cancer risk varies across populations, with approximately 1 in 30 North American adults displaying genetically-mediated radiation sensitivity15,17,18,19,20,21,22,23,24,25. Distinctly elevated risks from radon exposure are also observed in women and children11,12,13,26,27,28,29. Lifetime relative risk of lung cancer from radon is inversely proportionate with age, with the youngest being the most at risk due to innate pediatric radiosensitivity, faster breathing rates, lower body mass and, most potential years of life lost at time of exposure15,16,26. Thus, when considering cancer risks from radiation, it is essential to understand age at time of exposure.

## Results

### Residential building radon exposure and human occupancy data

The binary gender balance for the primary respondents was equal for those under 60, with a modest skew towards men for those 60 and older (Fig. 1D). The majority of participants report being in full (56.7%) or part (10.9%) time employment at the time of the survey; the remainder being retired (29.1%), unemployed (3.3%) or on disability leave (0.77%) (Fig. 1E). Of those in work, 34.3% reported doing so from home some or all of the time. Of the cohort providing demographic data, 1,128 participants also provided detailed occupancy data for their primary residence, which we then analyzed as a function of employment status (Fig. 1F). Those in work (or enrolled in formal education) spent an average of 5205 h/y at home, with those working-from-home, retired or on leave spending ~ 6200 h/y, and the unemployed spending 6726 h/y. Collectively, the entire cohort reported spending 5549 h/y inside their primary residence, which (allowing for time spent in other residences per year) fits well with data obtained from the Canadian and American National Human Activity Pattern Study32 that found the average adult spent 68.7% (6018 h/y) of life inside a residence.

To gain a sense of cumulative particle radiation exposure over the amount of time that each individual spent within the radon-tested environment, we next multiplied dose rates by the amount of time each person reported living full time in that property, and expressed these values as a function of the number of occupants. For the purposes of modelling, we operated on the assumption that radon levels within each property did not fluctuate significantly from year to year over the long term. This is supported by our previous work that found no year-on-year variations in the collective radon test outcome for North American large populations12, and the fact that all participants in this cohort verified that they had not engaged in any type of radon mitigation of their primary residence at any time before surveying. Estimated, cumulative particle radiation doses were an average of 40–50 mSv, with 23.4% of the population receiving ≥ 100 mSv, and 1.2% receiving ≥ 500 mSv to the lungs over the long term from residential radon inhalation (Fig. 2E). To estimate mSv/y particle radiation dose rates for a much larger population, we used the average amount of time our cohort spent in their primary residence per year (5549 h/y), and our larger radon test dataset as in Fig. 1A. The geometric mean outcome of 4.02 mSv/y compared well with the outcomes of our sub-study cohort in Fig. 2C, with the overall maximum dose rate from this larger population being 421.3 mSv/y (Fig. 2F).

### Residential Radon exposure by age of property and occupant demographics

We next sorted all participants based on the construction year of their specific residence, ranging from 1900 to 2020, and linked this to the average long term radon test outcomes (light grey bars, right y-axis) of those properties (Fig. 3A-B). As observed previously12,13, newer North American properties contained greater radon gas, with those constructed in the twenty-first century having the highest levels. We then analyzed these data as a function of the primary homeowner or renter’s reported age (dark grey line with dashed red polynomial trendline, left y-axis) (Fig. 3A), or the total number of occupants reported per residence (dark grey line with dashed orange polynomial trendline, left y-axis) (Fig. 3B). There was a clear trend wherein the newest properties had the highest radon and also the greatest number of occupants of the youngest overall ages.

All observations, coupled with geometric mean particle radiation dose rates, are summarized in Table 1. Construction periods were grouped into four 15 year brackets going back to 1960, with older properties (1900–1960) in a fifth bracket. People in properties built from 2006 to 2020 had the highest documented annual particle radiation exposure (to lung) dose rate, at 5.01 mSv/y, the most starkly contrasted to those in properties built from 1976 to 1990, who experience the substantially lower 3.60 mSv/y. While there was no statistically significant difference (p > 0.05) in the geometric mean age of occupants living in properties constructed from 1900 to 2005 (~ 53 ± 1.3 y), those in properties built within the past 15 y (2006–2020) were significantly (p < 0.0001) younger being a geometric mean age of 46.1 (CI95% [45.3, 46.9]) (Table 1, Fig. 3C). Properties from this period were also home to a significantly (p < 0.0001) greater number of occupants relative to all other property build periods (Table 1, Fig. 3D). Finally, we examined how many minors (ages 0–17) or pregnant occupants were reported to live full time in each property by the primary respondent (homeowner / renter), as a function of year of construction (Fig. 3E). People in the newest properties were the most likely to be caring for a first and/or second child, were the least likely to have no children present at all, and reported more than double the number of full time occupants who were pregnant compared to all other property ages.

## Discussion

The age-related home occupancy biases we observe are likely explained by data demonstrating that North American first time home-seekers (i.e. ages 24–44) have more limited financial resources that preclude buying or renting the typically more expensive residential properties in older and “more established” neighbourhoods, and/or are preferring newer properties with more modern design trends that are also smart-home ready and/or energy efficient37,38,39. This phenomenon is exemplified by the proliferation of suburban sprawl, ‘bedroom communities’ and ‘commuter towns’ across North America to meet the growing housing demands of younger people priced out major metropolitan city centres39,40,41. This is a rising public health issue, as 70% of the housing stock necessary to deliver on 2050 population growth projections has yet to be built42,43. The regional burden of radon-attributable lung cancer will only worsen if, as it is now, new housing stock continues to be built with an increasingly higher latent radon risk.

In terms of potential limitations, our sampling through citizen science enrollment was untargeted and, as we accepted all valid data from adult homeowners or renters, we consider selection biases to be minimized to the greatest possible extent. We acknowledge that our sampling methods may under-represent some in lower socioeconomic brackets, who may be less likely to prioritize radon test expenditures. As North American radon risks cannot yet be predicted accurately by property type, we consider it unlikely that our cohort is biased towards low or high radon-containing properties based on a willingness to participate. An essential future direction will be to consider the large-scale changes in residential occupancy (i.e. time spent at home versus at an office) elicited by the COVID-19 global pandemic, as well as general long term shifts towards working-from-home already occurring across workplaces. This information, weighted by age-of-exposure, should also serve as a future basis for long term radon-attributable lung cancer burden calculations.

## Methods

### Consent, study design and participant eligibility

All methods were carried out in accordance with protocols approved by Research Ethics Boards (REBs), adhering to recommendations for research involving citizen science participants31. Study design, methods and data analysis were pre-approved by either the Conjoint Health Research Ethics Board, Research Services, University of Calgary (IDs = REB17-2239, REB19-1522, REB20-1729) or the Health Research Ethics Board of Alberta, Cancer Committee, and conferred (HREBA.CC-17-0246), as the ‘Evict Radon’ national study and sub-studies thereof. Records of informed consent were obtained in all cases. The survey region included all of Canada, with data in this study encompassing residential properties and occupants from: Yukon Territory, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec and Atlantic Provinces spread across > 250 distinct cities, towns and rural districts. The study operated using random, convenience recruitment (any who wanted to join). Recruitment methods included print media, public seminar, online (website and social media) and mass media messaging via organic (unpaid) TV/radio exposure in an untargeted manner. No data from any constituent part of this cohort were from known or pre-selected lung cancer cases. Homeowners and renters of any building type were equally eligible. Commercial offices or hospitality service buildings were not considered. Participants were permitted to withdraw at any time.

### Surveying

From 2015 to 020, Canadians purchased alpha track 90 + day radon detectors that they then deployed, returned for analysis, and later received their specific radon reading in a confidential manner. Non-profit study kits were inexpensive (\$52), and were not considered to be a significant economic barrier to participation. Selection biases were considered, and socio-demographics adjusted for in analyses to the fullest extent. Following consent and placement of a radon test, all participants were invited to complete separate building metric, demographic and residential occupancy surveys using either Qualtrics or Hosted-in-Canada survey platforms. Of 18,971 invited participants from the ‘whole cohort’, all provided building metric data, and 3,518 in the ‘sub-study cohort’ additionally consented to provide and return detailed residence occupancy and/or demographic data. The relevant survey questions used to generate data for this particular study are outlined in Supplemental File 1.

To convert Bq/m3 indoor air radon levels to human mSv radiation exposures (to lungs), the ICRP provides the following formula44:

$$\left( {6.7 \times 10^{ - 6} \;{\text{mSv }}per{\text{ Bq}}\;{\text{h}}/{\text{m}}^{3} } \right) \, \times \, \left( {\left[ {radon} \right]{\text{ Bq}}/{\text{m}}^{3} } \right) \, \times \, \left( {\left[ {time\; \, in\; \, residence\; \, per\; \, year} \right]{\text{ h}}/{\text{y}}} \right) \, = {\text{ mSv}}/{\text{y}}$$

Values for the amount of time spent in the primary residence per year for a typical adult (“time in residence per year”) were calculated from individually reported residential occupancy data from 1,128 participants, and cross-referenced with data from the National Human Activity Pattern Study (NHAPS)32. The NHAPS, which included responses from both Canadian and American respondents, estimated that 68.7% of life was spent inside a residence for the average adult. Of the total 8760 h per year, this equates to 6018 h/y and represents an average of all different employment statuses. Participants reported their data by season (Winter, Spring, Summer, Fall), with weekend/holiday versus workdays accounted for within the questionnaires (see Supplemental File 1). All response-derived “time in residence per year” outcomes were linked to individually-reported employment statuses and used to extrapolate the same values for all remaining participants (2,390) for which employment data was collected. Please note that for the purposes of participant responses in this survey, being enrolled in full or part time formal education (at a university, college, technical school, etc.) was considered to be a type of employment and grouped with those responses.

### Statistical analysis

Statistical analysis was carried out using Excel, Prism and R (4.0.2). ANOVAs were carried out to test differences between groups (e.g. year of construction, occupant age, mSv, etc.), with Bonferroni-Holm post-hoc testing carried out to characterize group differences for pairwise comparisons if the ANOVA reached significance.

### Data availability

The de-identified raw data sets generated by the current study are available to academic researchers at public institutions following reasonable request to the corresponding author, and will require a data transfer agreement. Data may not be used for private, commercial, or for-profit purposes for any reason.