Certain Cool Roofing Materials Could Increase Air Pollution

A new study reveals that Researchers at the South Coast Air Quality Management District and the University of Southern California have discovered that an extensive installation of certain "cool roof" materials in the region could increase ozone and fine particulate pollution levels up to some extent.

The study describes that comprehensive performance standards are required for cool-roofing materials in order to help decrease or avoid the predicted increase in ozone levels. Yet, small increases in fine particulate (PM2.5) levels are predicted to occur with or without improved standards.

Cool roofs have many benefits including reduced energy use for cooling and mitigation of the significant health impacts of heat waves. While the future, widespread use of certain kinds of cool roofing materials could slightly increase air pollution levels, we in no way want to discourage this technology. This study shows what needs to be done to help cool our cities and avoid increasing ozone levels as an unintended consequence.

Wayne Nastri, Executive Officer, SCAQMD

The research, titled "Air Quality Implications of Widespread Adoption of Cool Roofs on Ozone and Particulate Matter in Southern California," was published in the Proceedings of the National Academy of Sciences.

The scientific paper was submitted by a team of SCAQMD Scientists headed by Scott A. Epstein, Ph.D., in association with George Ban-Weiss, Ph.D., a Professor of Civil and Environmental Engineering at USC. They used state-of-the-art air quality computer models and meteorology, measurements of cool roofing materials and comprehensive databases of the region's rooftops to predict the air quality impacts of the increasing use of cool roof materials - usually light-colored and extremely reflective - that are likely to result from present statewide energy efficiency standards.

A number of scientific studies have established that when darker roofs and building materials are replaced with extremely reflective materials, then peak daytime temperatures can be reduced and the so-called "urban heat island effect", where cities can be several degrees warmer than less-urbanized surrounding areas, can be mitigated.

Many cool-roofing materials however reflect more ultraviolet light (UV) than their conventional counterparts, leading to a rise in ozone formation. UV rays induce smog formation on their way down to Earth. The cool roofs cause a "double-shot" of ozone formation when they bounce back up into the atmosphere.

This increase in ozone can be avoided if a complete roofing standard is adopted to prevent the overall UV reflectance of newly-installed cool roofs from increasing. Cool roofing materials that reflect the same amount or even less UV than conventional roofing materials are available these days.

This study highlights the importance of considering how strategies used to mitigate one environmental issue can have co-benefits and/or unintended consequences on other environmental systems. Whether air pollution improves or worsens from cool roof installations depend on a host of competing chemical and meteorological factors. Given that our study focuses on the Los Angeles basin, future research is needed to investigate how these competing processes dictate air pollution impacts in cities around California and beyond.

George Ban-Weiss, Ph.D., Professor of Civil and Environmental Engineering, USC

According to the study, while a change in cool roofing materials can mitigate an increase in ozone formation, the predicted small increase in PM2.5 levels is because of the overall cooler surface temperatures resulting in lower inversion layers and weaker sea breezes, and will occur irrespective of UV reflectance of roofing materials.

The work also concluded that the installation of cool paving materials could have an even larger impact on ozone and PM2.5 levels as the amount of area paved in the region is considerably larger than the total roofing area affected by state energy efficiency standards.