The short-term effect of residential home energy retrofits on indoor air quality and microbial exposure: A case-control study

Weatherization of residential homes is a widespread procedure to retrofit older homes to improve the energy efficiency by reducing building leakage. Several studies have evaluated the effect of weatherization on indoor pollutants, such as formaldehyde, radon, and indoor particulates, but few studies have evaluated the effect of weatherization on indoor microbial exposure. Here, we monitored indoor pollutants and bacterial communities during reductions in building leakage for weatherized single-family residential homes in New York State and compared the data to non-weatherized homes. Nine weatherized and eleven non-weatherized single-family homes in Tompkins County, New York were sampled twice: before and after the weatherization procedures for case homes, and at least 3 months apart for control homes that were not weatherized. We found that weatherization efforts led to a significant increase in radon levels, a shift in indoor microbial community, and a warmer and less humid indoor environment. In addition, we found that changes in indoor airborne bacterial load after weatherization were more sensitive to shifts in season, whereas indoor radon levels were more sensitive to ventilation rates.

Recording environmental conditions and radon concentration 173 Both indoor and outdoor environmental conditions were recorded during each sampling period.
174 Indoor temperature and relative humidity in 11 locations inside the house was recorded every 175 min with temperature and relative humidity loggers (HOBO UX100-003 Temperature-Relative 330 Indoor plants were present in 6 out of 9 case homes, while present in all control homes. Three 331 case homes and 1 control home were occupied by people who were diagnosed with asthma, 332 while 6 case homes and 7 control homes had occupants with environmental allergies ( Table 1).
333 In addition, in 7 case homes and 7 control homes, occupants reported having respiratory 334 symptoms when inside the home, including: sinusitis, intermittent sneezing, and sore-dry throat 335 (Table 1). However, no significant differences in basement type, indoor plants, asthma, allergies, 336 and respiratory symptoms were found between case and control homes ( Table 1).

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338 Air-sealing effectiveness of weatherization 339 In the 9 case homes, improved air sealing after weatherization resulted in an average total 340 decrease in building leakage of 1690 m 3 h -1 (±887 m 3 h -1 ) ( Table 1), corresponding to an average 341 decrease of 21.8% (±9.68%) from the pre-retrofit building leakage measurement (ordered from 342 low to high in Fig 1A). When these building leakage values from blower-door tests were 343 corrected for the house volume, the order of impact changed, with an average decrease in ACH 50 344 of 0.7 h -1 (Fig 1A). Natural ventilation rate during the sampling period was measured via CO 2 345 tracer gas decay (ACH CO2 ). For the combined data points, the difference in natural ventilation 346 rate for the 9 case homes between the sampling events was significantly more negative than the 347 control homes (Fig 1B), showing again a measured effect of weatherization. The natural 348 ventilation rate for each individual case home either decreased or remained the same after 349 weatherization, with an average decrease in ACH CO2 of 0.7-air change per h (Fig 1B). Natural 350 ventilation for control homes fluctuated within a ±1-air change per h, with an average of 0.2-air 351 change per h for all data points during the two sampling events, but did not differ (Fig 1B). This 352 reduction in natural ventilation rates in case homes correlated with an increase in indoor-outdoor 353 temperature ratio (S1 Table). Indeed, we found that weatherization resulted in a significant 354 increase in indoor-outdoor temperature ratio, as well as a significant decrease in indoor-outdoor 355 relative humidity ratio (Fig 1C,D). These results showed that weatherization reduced the 356 leakiness and natural ventilation rates of older homes in Tompkins County, NY; leading to 357 improved shielding from outdoor weather.   Table 1). Interviews with occupants of only the case homes indicated a significant increase in 384 thermal comfort ( Table 1) and noticeably lower heating requirements (data not shown). We did 385 not observe significant differences in basement and living-area radon levels between case and 386 control homes for the first sampling period when the data was averaged (Figs 2A,C).
387 Weatherization increased the basement radon levels for the case homes, while no significant 388 change in the average basement radon level in control homes was observed between the first and 389 second sampling event (Fig 2A). No significant difference for any of the comparisons in average 390 living-area radon levels was observed (Fig 2C). We then re-analyzed the same radon level data 391 by calculating the difference in radon levels between the first and second sampling periods for 392 each individual home (Figs 2B,D). Again, we found that the average differences in basement 393 radon levels in case homes were significantly greater than those in control homes (Fig 2B). The 394 average difference in radon levels between the first and second sampling periods in the living 395 area was also significantly greater in case homes compared to control homes (Fig 2D). Thus, we 396 observed that radon levels increased during weatherization of these older homes in Tompkins 397 County, NY.     (Fig 4; S8 Fig).  507 To determine environmental factors that drive the indoor airborne microbiome composition, we 508 consolidated all indoor air samples from case and control homes. Next, constrained ordination 509 was utilized to correlate microbiome composition to environmental factors via multiple linear 510 regression. We maintained the samples separated by the particulate size bins, and therefore 511 would include factors that not only drive compositional changes in the airborne bacteria 512 community between sampling periods, but also factors that can potentially drive compositional 513 difference between particle size bins, such as evaporation and condensation. However, we did 514 not observe other factors than the environmental factors to drive the microbial composition. We 515 included these changes in environmental factors in the ordination: indoor-outdoor temperature; 516 indoor-outdoor relative humidity; occupancy rate; occupant density; percent the house is 517 occupied; ventilation rate; average wind speed; and average rain-snow. Of these, four 518 environmental conditions were found to significantly drive the changes in the indoor airborne 519 bacteria composition (in the order of importance): 1) occupant density; 2) ACH 50 ; 3) indoor 520 temperature; and 4) indoor relative humidity (Fig 5). 548 by comparing changes in case homes to baseline changes in control homes that did not undergo 549 weatherization between sampling periods. Weatherization led to a reduction in building leakage 550 by an average of 22 percent (Fig 1). This is similar to previous evaluations of weatherization 551 effectiveness across the United States (5). After weatherization, a significant number of families 552 reported being thermally more comfortable, which was reflected quantitatively by an increase in 553 indoor-outdoor temperature ratio and a decrease in indoor-outdoor relative humidity ratio after 554 weatherization (Fig 1). Although occupants reported more thermal comfort, there was no 555 noticeable change in respiratory symptoms or exacerbation of pre-existing symptoms when 556 inside the house. Our work agrees with larger studies (5, 46, 47), but our result is only 557 representative of health effects after a relatively short period of several months.  48) found that the radon level was weakly correlated with a decreased 563 natural ventilation rate. We found that both basement and living area radon levels were 564 significantly increased in case homes after weatherization when compared to baseline 565 fluctuations in control homes. In the living area, where occupants spend the majority of their 566 time, we observed minimal weatherization effect on radon levels in most case homes. For two 567 case homes, however, we observed an exception because the radon levels exceeded the EPA 568 action level of 4.0 pCi L -1 after weatherization (Fig 2). These two homes had different building 569 leakage decreases of 14% and 30%, which indicates that living area radon concentration was not 570 strictly correlated with weatherization effectiveness. Therefore, it is necessary for homeowners 571 to measure the radon levels in a home after weatherization, regardless of the extent of the retrofit 572 performed because each building envelope results in different indoor air quality outcomes after 573 weatherization. 574 575 In multiple epidemiological studies, airborne particulate matter concentrations have been 576 implicated as an important determinant of human health (49). A previous study found that 577 particulates with a relatively small particulate matter size (PM 3 ) were weakly correlated with a 578 decrease in natural ventilation after weatherization (48). However, we did not observe this 579 finding. For our study, the average IO mass ratio in both case and control homes were greater 580 than one, indicating significant indoor contribution to occupant exposure (S2 Fig), which would 581 have masked the contribution of outdoor air to the indoor air mass concentration. Therefore, the 582 change in outside particulates played a smaller role on the change in inside particulates due to 583 weatherization. Instead, we found a significant correlation between changes in indoor mass 584 concentrations and changes in occupancy rate. However, we did not record the occupant activity S1 ∆ IO RH Gradient 0.05 -0.01 0.08 0.22 0.84*** -0.79*** *: p < 0.05; **: p < 0.01; ***: p < 0.001 ∆: defined as value at second sampling event minus value at first sampling event ACH CO2 : natural ventilation via CO 2 tracer gas decay IO Temperature Gradient: ratio of the indoor and outdoor (IO) temperatures, calculated as a time average throughout the sampling period IO RH Gradient: ratio of the indoor and outdoor (IO) relative humidity, calculated as a time average throughout the sampling period