Leafing Las Vegas: Health dangers of city plants revealed

Kate Ravilious, New Scientist 18 Jun 09;

Tree-lined avenues and flower-filled central reservations are pretty, but they could be damaging your health. A new study shows that some plants can increase the rate of ozone production by up to 50 times.

Certain plants, such as rosemary, juniper and pine trees, emit chemical compounds known as terpenes, thought to help deter insect predators, or protect the plant from other stresses like high temperatures.

However, when terpenes mix with pollutants (particularly nitrogen oxides from industry and traffic) they react to produce ozone – a key ingredient of photochemical smogs, and a health hazard that can cause breathing difficulties and may cause cancer.

Scientists have long known that terpenes from forests can cause a significant rise in ozone levels when they mix with air pollution. But could smaller clusters of plants in urban environments be damaging too?

Mark Potosnak, from DePaul University in Chicago, Illinois, and his colleagues measured terpene emissions from plants lining the sidewalks of Las Vegas. They also studied air quality – including levels of nitrogen oxides and ozone – in a number of central and suburban regions.
Planted perils

In all cases, ozone levels exceeded the US Environmental Protection Agency safe standard (an average of 75 parts per billion over an eight-hour period) and in the worst case reached 107ppb.

Modelling the data revealed that the mix of terpenes and pollutants was responsible for a significant rise in ozone levels – boosting production rates by up to 50 times – particularly downwind of the plants, in suburban neighbourhoods. "It's surprising because Las Vegas has relatively little urban vegetation," says Potosnak.

All over the world hot cities are likely to have similar problems, Potosnak thinks. But the situation can be easily remedied by choosing plants carefully. "Some plant species are very low emitters. Shoestring acacia is a great plant: low water use and low terpene emissions," says Potosnak.

Journal reference: Atmospheric Environment (in press)