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This issue is especially relevant since we spend a great portion of our time indoors.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Objective<\/jats:title>\n                    <jats:p>\n                      To develop a two-stage model to estimate indoor nitrogen dioxide (NO\n                      <jats:sub>2<\/jats:sub>\n                      ) concentrations.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>\n                      The study was conducted in the Barcelona Life Study Cohort (2018\u20132021), a cohort of pregnant individuals in Barcelona, Spain. Indoor and outdoor NO\n                      <jats:sub>2<\/jats:sub>\n                      concentrations were measured using passive NO\n                      <jats:sub>2<\/jats:sub>\n                      samplers placed in participants\u2019 homes at weeks 12 and 32 of pregnancy. A total of 1695 indoor measurements and 1577 outdoor measurements were collected. In the first stage, we modeled the indoor-outdoor (I\/O) NO\n                      <jats:sub>2<\/jats:sub>\n                      ratio as a proxy for infiltration using a linear mixed-effects model with repeated measures using 1528 1-week integrated I\/O NO\n                      <jats:sub>2<\/jats:sub>\n                      measurements. In the second stage, we used a random forest algorithm to estimate weekly indoor NO\n                      <jats:sub>2<\/jats:sub>\n                      concentrations covering the full pregnancy period, incorporating the predicted I\/O ratios, a previously developed hybrid-model outdoor NO\n                      <jats:sub>2<\/jats:sub>\n                      estimate, meteorological data, and home characteristics.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      The I\/O NO\n                      <jats:sub>2<\/jats:sub>\n                      ratio model showed moderate performance, with a cross-validated R\n                      <jats:sup>2<\/jats:sup>\n                      of 0.27 under leave-one-subject-out (generalized model) validation and 0.70 under leave-one-observation-out validation (cohort performance). The indoor NO\n                      <jats:sub>2<\/jats:sub>\n                      model achieved a 10-fold cross-validated R\n                      <jats:sup>2<\/jats:sup>\n                      of 0.53 and RMSE of 1.28\u2009\u00b5g\/m\n                      <jats:sup>3<\/jats:sup>\n                      . Finally, we estimated indoor NO\n                      <jats:sub>2<\/jats:sub>\n                      concentrations for each week of pregnancy using a quantile random forest (QRF) algorithm incorporating uncertainty estimation through prediction intervals.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Significance<\/jats:title>\n                    <jats:p>\n                      The findings demonstrate that accounting for dynamic infiltration processes and individual-level behaviors improves the estimation of indoor NO\n                      <jats:sub>2<\/jats:sub>\n                      exposure.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Impact<\/jats:title>\n                    <jats:p>\n                      Integrating indoor and outdoor NO\n                      <jats:sub>2<\/jats:sub>\n                      measurements in a two-stage modeling framework and combining it with key indoor determinants improves indoor NO\n                      <jats:sub>2<\/jats:sub>\n                      estimation.\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1038\/s41370-026-00969-1","type":"journal-article","created":{"date-parts":[[2026,9,2]],"date-time":"2026-09-02T10:16:01Z","timestamp":1788344161000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["A two-stage modeling approach to estimate indoor NO2 exposure: a Barcelona case study"],"prefix":"10.1038","author":[{"ORCID":"https:\/\/orcid.org\/0009-0000-7753-0227","authenticated-orcid":false,"given":"Alan","family":"Dom\u00ednguez","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yu","family":"Zhao","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Karl","family":"Samuelsson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Pol","family":"Jimenez-Arenas","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marta","family":"Cirach","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Gustavo","family":"Arevalo","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lluis","family":"Barril","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jose","family":"Lao","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Maria","family":"Foraster","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mirea","family":"Gasc\u00f3n","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cecilia","family":"Persavento","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mark","family":"Nieuwenhuijsen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cathryn","family":"Tonne","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Teresa","family":"Moreno","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xavier","family":"Querol","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michael","family":"Jerret","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Joel","family":"Schwartz","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jordi","family":"Sunyer","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Payam","family":"Dadvand","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ioar","family":"Rivas","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xavier","family":"Basaga\u00f1a","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,9,2]]},"reference":[{"key":"969_CR1","unstructured":"Health Effects Institute. 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All enrolled pregnant participants in this study signed an informed consent form. The ethical approvals were obtained from the Clinical Research Ethics Committee of the Parc Salut Mar (2018\/8050\/I), the Medical Research Committee of the Fundaci\u00f3 de Gesti\u00f3 Sanit\u00e0ria del Hospital de la Santa Creu i Sant Pau de Barcelona (EC\/18\/206\/5272), and the Ethics Committee of the Fundaci\u00f3 Sant Joan de D\u00e9u (PIC-27-18).","order":2,"name":"Ethics","label":"Ethical approval","group":{"name":"EthicsHeading","label":"Ethics"}}]}}