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Indoor radon is a major contributor to natural radiation exposure and is recognized as the second leading cause of lung cancer worldwide. It originates from the decay of uranium and thorium naturally present in soil and building materials and can enter buildings through cracks and structural openings. This study aimed to measure indoor radon concentrations in residences located in Carambeí and Curitiba (Santa Felicidade district), Paraná, Brazil, and to compare the results with international radiological protection recommendations. Passive CR-39 solid-state nuclear track detectors were deployed for a minimum exposure period of 90 days. After exposure, track densities were analyzed by optical microscopy using a previously validated methodology developed through collaboration between UTFPR and CDTN/CNEN. In Carambeí, 97% of the monitored residences showed radon concentrations ranging from 95 ± 15 Bq/m³ to 171 ± 25 Bq/m³, remaining below the reference level recommended by the International Commission on Radiological Protection (300 Bq/m³). However, one residence presented a concentration of 461 ± 65 Bq/m³, exceeding international guidelines and indicating the need for mitigation measures. In Curitiba, concentrations ranged from 87 ± 14 Bq/m³ to 143 ± 22 Bq/m³, all below recommended limits. The results demonstrate regional variability associated with local geological characteristics and highlight the importance of systematic radon monitoring. These findings contribute to the assessment of indoor exposure in southern Brazil and support the development of national policies for radon control and public health protection.