We thank Olusanya et al. for their thoughtful comments1 and for the opportunity to clarify the scope and implications of our study.2 We also take this opportunity to acknowledge Dr Olusanya’s outstanding contribution to the prevention and treatment of severe neonatal hyperbilirubinemia in Africa and other low-resource settings. Her work has helped provide realistic therapeutic options for infants who might otherwise have no access to effective phototherapy.3,4

Our article did not intend to question the value of filtered sunlight phototherapy (FSPT) when it is delivered as a controlled medical intervention. On the contrary, we explicitly distinguished FSPT from the common empiric recommendation to place a jaundiced newborn “in the sun, behind a window” at home. The trials led by Olusanya et al used custom-designed canopies, spectrally characterised filters, hospital supervision, temperature monitoring and serial bilirubin measurements. This is fundamentally different from exposing a healthy newborn to sunlight through residential glazing, where irradiance, spectrum, duration, body surface exposed and thermal load are neither standardised nor measured.

This distinction is particularly important in Europe and in other high-income countries. In these settings, the clinical challenge is usually not the absence of electric phototherapy, but the persistence of informal advice that may give families a false sense of safety and delay appropriate bilirubin measurement, follow-up or conventional treatment.5 Our experimental results showed that common residential glazing transmits high levels of visible blue light, together with relevant ultraviolet A and infrared radiation, without spectral selectivity or dose control. Therefore, sunlight behind residential glass should not be presented as a preventive or therapeutic strategy for neonatal jaundice where safe, regulated alternatives are available.

We fully agree that the global reality is heterogeneous. In settings where electric phototherapy is unavailable, unreliable, or unaffordable, the ethical dilemma may not concern FSPT versus ideal hospital-based phototherapy, but a controlled filtered-sunlight system versus unfiltered sunlight exposure or no treatment at all. In that context, FSPT is a valuable, pragmatic and potentially life-saving innovation.

The challenge now is to translate this concept into safe, affordable and scalable solutions. There is a clear unmet need for home-based phototherapy devices that are inexpensive, robust, spectrally appropriate, thermally safe and easy to monitor. Such devices should preserve the therapeutic blue-green band, block ultraviolet and excessive infrared radiation, allow dose control, and be integrated into clinical pathways that include bilirubin surveillance and parental education.

Thus, our message is not that sunlight can never be harnessed, but that it must not be improvised. In developed health care systems, domestic exposure behind ordinary windows should be discouraged. Future innovation should focus on controlled, low-cost phototherapy solutions that can safely meet the needs of families and health systems without replacing evidence-based screening and follow-up.