There is a traditional belief that moonlight can be used to bleach laundry linen. Because of the low intensity of moonlight compared to that of sunlight, this appears implausible in light of current scientific understanding. However, J.P. Parisot (1986) suggested that the bleaching effect is real and is caused by the presence of hydrogen peroxide (H₂O₂) in the dew, which is enhanced in cloudless sky conditions when the moon is visible. In this hypothesis, the moonlight would not be a cause but instead correlated with the presence of natural H₂O₂. This physical explanation is now widely used in support of the traditional belief.

To our knowledge, the reality of the effect has never been assessed with a proper scientific study and therefore has not been demonstrated. Whether the effect is real or not, the physical interpretation proposed by Parisot is unlikely, because the measured H₂O₂ concentrations in the atmosphere and natural dew are much lower (by several orders of magnitude) than those required to generate a bleaching effect.

A Traditional Belief

Many popular beliefs exist about the influence of the moon on agriculture, human sleep, and birth rate, although there is no solid evidence of such effects. In fact, the results of analysis using scientific methods often contradict the popular beliefs (see, for example, Morton-Pradhan et al. 2005; Beeson 1946; Mayoral et al. 2020). One of them concerns a traditional belief that moonlight will bleach laundry. It is notably attested by sayings such as the French La lune mange les rideaux (“The moon eats the curtains”) and in various online content in French and English (see sidebar). This lunar bleaching power is supposed to be superior to that of the sun, and it is therefore customary to spread laundry on the ground in the grass on evenings of the full moon to whiten it (Parisot 1986). This belief is notably invoked in the explanations of the traditional technique of “bleaching in the meadows” of Gérardmer in France, practiced since the eighteenth century until the first half of the twentieth century.  

A Possible Explanation

In 1985, J.P. Parisot proposed a scientific explanation for this belief: the bleaching effect could be caused by the presence of H₂O₂ in dew. People wrongly attributed the whitening effect to the moon, but it was actually due to the H₂O₂ naturally present in dew, which is enhanced when the sky is cloudless and the moon is more visible.

Parisot described the atmospheric chemical reactions leading to such H₂O₂ creation in the dew and made a theoretical calculation to evaluate its production (Parisot 1986). The paper does not claim to provide proof that things really happen this way; it simply suggests that this could be an explanation that remains to be confirmed.

With lunar laundry lightening understandably not the subject of major scientific interest, Parisot’s scientific publications remain unchallenged. His paper is not cited by any other peer-reviewed publications. The fact that the published paper was written in French is likely also a factor for its absence of impact on the scientific community.

But in the field of science popularization, presumed influences of the moon remain a subject of interest, and many French books, press articles, blogs, websites, and forums refer to this paper. This explanation is widely presented as valid because it is seen as one of the few beliefs associated with the moon that are partially true and explained by science rather than by our cognitive biases. This is notably the case in the French books Sous l’emprise de la Lune by Jérome Bellayer (2011) and Astronomie de l’étrange by Yaël Nazé (2021). In a dozen various online sources consulted, all refer to this single publication to justify the reality of the phenomenon (see sidebar).

A Concentration Issue

Here we analyze whether the H₂O₂-in-dew interpretation is valid. This chemical has been measured extensively, and typical concentrations are provided in published papers. H. Sakugawa and I.R. Kaplan (1993) find 10 to 90 µM (micro-mol per liter) in rain and clouds, but <1 µM in dew collected over mountains of Southern California. V. Ortiz et al. (2000) made extensive measurements in the vicinity of Santiago, Chile, and found an average concentration of 5.4 µM in rain and 2.2 µM in dew. K. Watanabe et al. (2009) reported concentrations in dew waters of a few ppm, although one sample collected in high altitude showed a value as high as 65 µM. These measurements show significant variability but clearly indicate that the typical concentration of H₂O₂ in dew is at most a few µM. As there are 56 moles of H₂O per liter, the relative concentration [H₂O₂]/[H₂O] is on the order of 10^-7, or one part in 10 million.

Parisot made an estimate of the H₂O₂ concentration that may be dissolved in water during a day and found 0.71 g m^‑2, though he did not extend his estimates to the potential concentration in the dew. Typical dew deposits at night are 100 to a few hundred g m^‑2 (e.g., Jacobs et al. 1999; Andrade 2003), so the Parisot estimate is equivalent to a [H₂O₂]/[H₂O] concentration of 10^‑2 to 10^‑3, much larger than that of actual measurements.

Using “artificial” H₂O₂ for bleaching laundry used to be a common practice that remains recommended in books and on internet sites. For this purpose, the recommended concentration of H₂O₂ to be included in the water varies between 10^‑1 and 10^‑2.  Even the 10^‑2 concentration is five orders of magnitude larger than what is found in typical dew and four orders of magnitude larger than the highest natural concentration recorded. No effect is expected at these concentrations. Clearly, the fact that H₂O₂ is used to whiten laundry can hardly be used to justify that the same process is occurring with dew, because the concentrations in dew are so much lower. The concentration estimates made by Parisot would have made the process plausible, but the practical effects do not. 

To our knowledge, there is no rigorous experimental demonstration that laundry tends to whiten when exposed to moonlight, despite countless testimonials to the contrary.  

References

Andrade, J.L. 2003. Dew deposition on epiphytic bromeliad leaves: An important event in a Mexican tropical dry deciduous forest. Journal of Tropical Ecology 19(5): 479–488.

Beeson, C.F.C. 1946. The moon and plant growth. Nature 158: 572–573.

Jacobs, A.F.G, B.G. Heusinkveld, and S.M. Berkowicz. 1999. Dew deposition and drying in a desert system: A simple simulation model. Journal of Arid Environments 42: 211–222.

Ortiz, V., M.A. Rubio, and E. Lissi. 2000. Hydrogen peroxide deposition and decomposition in rain and dew waters. Atmospheric Environment 34: 1139–1146.

Mayoral, O., J. Solbes, J. Cantó, et al. 2020. What has been thought and taught on the lunar influence on plants in agriculture? Perspective from physics and biology. Agronomy 10(7): 955.

Morton-Pradhan, S., R. Curtis Bay, and D.V. Coonrod. 2005. Birth rate and its correlation with the lunar cycle and specific atmospheric conditions. American Journal of Obstetrics and Gynecology 192(6): 1761–2184.

Parisot, J.P. 1986. La lune mange-t-elle les couleurs? Earth Moon Planet 34: 273–279.

Sakugawa, H., and I.R. Kaplan. 1993. Comparison of H₂O₂ and O₃ content in atmospheric samples in the San Bernardino mountains, Southern California. Atmospheric Environment. Part A. General Topics 27(9): 1509–1515.

Watanabe, K., et al. 2009. Measurements of peroxide concentrations in precipitation and dew water in Toyama, Japan. Bulletin of Glaciological Research 27: 1–5.