In 2019, scientists tested a diaper-like superabsorbent polymer in lab-made asphalt samples; the water-storing material cooled the specimens by up to 12°C

If you have ever asked yourself why the cities seem so much hotter than the surrounding area in the countryside despite being only a few miles away, well, the answer is right beneath your feet. Cities consist of asphalt roads and huge parking lots, and they absorb a tremendous amount of heat but then do a horrible job of releasing it. This is exactly what a group of scientists sought to address in the course of their 2019 research paper titled "The Performance of Super Absorbent Polymer (SAP) Water-Retaining Asphalt Mixture," published in the journal Materials.

The researchers at Chang'an University in Xi'an, China, created an asphalt composite made of superabsorbent polymers (SAPs). SAPs are the same group of water-retaining polymers that help to keep babies dry in their diapers. In their laboratory tests, when this mixture was added to porous asphalt test samples, the mortar-filled asphalt soaked up water, and its temperature dropped by 12°C (or roughly 21.6°F), due to evaporation of the absorbed water. Who knew the secret to cooler streets might be the same material stuffed inside a diaper, even if this was a laboratory specimen test, not a road that has ever been laid down and driven on.

Why your city feels like a frying pan


This is not just an interesting piece of information for your game nights; rather, it is a serious problem for the USA. As stated by the Environmental Protection Agency, typical pavements can get as hot as 120 to 150°F under the blazing sun in summertime. Moreover, urban environments tend to be a few degrees warmer than the adjacent land in many places all around the USA due to what is called the urban heat island effect. Climate change makes humid, heat-heavy cities like Houston and Atlanta face even more dangerous heat waves, threatening the lives of outdoor workers, children, as well as seniors: the kind of water-rich climates where a water-retaining pavement could realistically stay damp enough to work.

So far, most cities in America have opted for the apparent solution: use lighter pavement that will reflect the sunbeams instead of absorbing them. It has been tested by the city of Los Angeles within its "cool pavement" coatings program. However, according to a 2020 study titled "Solar reflective pavements—A policy panacea to heat mitigation?" by researchers at UCLA and Arizona State University, reflective coatings only reflect the heat to the sides and up in the air, which means that while the pavement gets cooler, the pedestrians become warmer. SAP asphalt follows a completely different approach. It does not reflect the heat but makes it evaporate away as sweat does from you.

How a diaper ingredient ends up in a highway

The super absorbent polymers (SAPs) are made from sodium polyacrylates, which have an amazing property of absorbing hundreds of times their own weight in water; hence their presence in products like diapers, pads, and soil additives for drought-prone land is of great importance. As explained in the Materials study titled the researchers simply added SAP to the cementitious mortar and filled the pores of porous asphalt with it.

Water enters through the surface and gets absorbed by the SAP, which takes only a few days to evaporate while extracting the heat from the surface in the process. Not only that, but this material also seems mechanically robust since the study found it to have good rut resistance and moisture durability, meaning that it won't crumble under the pressure of traffic.

As it turns out, this isn't the only study supporting this conclusion. Another study titled “Cooling water-retentive semi-flexible pavement with light-colored grout and recycled materials,” conducted in 2025 and published in the journal Construction and Building Materials, developed its own water-retentive pavement and noted that pavement incorporating SAP into alkali-activated slag resulted in an impressive 13.3°C temperature reduction, a result that echoes the 2019 findings six years on, using a different formulation.

A promising fix, not a silver bullet

But, before anyone begins to fantasize about asphalt diapers being rolled out across every single American interstate, a note of realism is due in place. This innovation has been tried only in controlled laboratory settings by research teams based in China, and the cooling effect is strongly dependent upon how frequently the pavement gets moistened. That dependence is the technology's biggest catch: it works best in the humid, hot cities mentioned earlier, but it offers little practical use in dry Western states like Arizona, which are already under severe water rationing and are arguably in the most urgent need of a cooling fix.

However, the idea behind this innovation is indeed very interesting and serves as a good illustration of the fact that sometimes the most efficient solutions may be quite ordinary, like the material that is currently being used in diapers can be used for reducing heat on roads. As long as urban cities continue looking for ways to cope with increasingly hot summer temperatures, there is always a possibility that such unexpected innovations will keep appearing.