Wind whisks lead across the Pacific Ocean to California.

Nov 18, 2010


Ewing, SA, JN Christensen, ST Brown, RA Vancuren, SS Cliff and DJ DePaolo. 2010. Lead isotopes as an indicator of the Asian contribution to particulate air pollution in urban California. Environmental Science and Technology



Synopsis by Emily Barrett and Wendy Hessler



A new study finds lead from Asia in California air samples, providing evidence that wind can transport airborne pollutants across continents and oceans. This new research is further proof that air pollution is a global issue and needs international cooperation to reduce environmental and health impacts. Approximately one-third of the lead found in the air samples taken from sites in the San Francisco Bay area originated in Asia, but the fraction varied by season and weather patterns. It is likely that other contaminants originating in Asia may reach the U.S. in the same way.



Air pollution is a concern because of its environmental impact as well as its human health risks. Most air pollution is associated with human activity, particularly burning fuel for manufacturing and transportation. In light of this, many governments have made concerted efforts to reduce emissions, from imposing limits on industrial output to encouraging use of mass transportation.

Even as Western nations try to curb emissions, rapid industrial expansion and growing markets elsewhere – particularly in Asia – provide new sources of air pollution. The impact is often looked at primarily on a local level. For instance, there was extensive debate over how the poor air quality in Beijing, China, might affect athletes at the 2008 Summer Olympic Games.

Unfortunately, growing evidence suggests that industrial pollution is anything but a local issue. Global air currents can actually transport pollutants quite far from their original sources, causing environmental and health problems even at a distance.

One such pollutant is lead, which is commonly released into the atmosphere from coal combustion and other industrial processes. Lead can contribute to atmospheric and climate changes by altering sunlight's reflection and absorption patterns as well as atmospheric ice formation. 

Lead can also directly affect health. Lead poisoning is a serious condition that can end in death. Exposure to lower levels can harm the nervous system and affect memory, reproduction, behavior and blood pressure. Children are particularly vulnerable to its effects because of their small size and rapidly developing brain and nervous systems. Local and national measures designed to limit lead exposure include removing it from gasoline, paint, jewelry and other products and cleaning up lead-polluted industrial and residential sites.

These measures may be insufficient, however, because airborne lead emitted from industrial sources – such as burning and mining coal and metal ores – can spread widely across the globe. For example, lead levels in the northern Pacific Ocean and Canada's ice have been increasing over the last two decades.

This migration means that local and regional efforts may not be sufficient to protect health. A better understanding of how and to what extent airborne pollutants, including lead, migrate in air currents is critical for making effective policy to protect health and the environment.

What did they do?

Particles in air samples were collected between December 2007 and May 2008 at two sites in the San Francisco Bay area. The sites represented Asian and local sources of lead pollution. One site – Mt. Tamalpais – is relatively rural and coastal and likely to be exposed to Pacific Ocean air currents that carry pollution from overseas. The other – Chabot Observatory – is an inland, urban location that would reflect lead from local industrial sources. 

The particle samples were analyzed for the amounts and types of lead atoms they contained. Air samples collected years earlier – between 2003 and 2005 from sites in inland California and in 2002 from China – were also analyzed for their lead type and content.

Lead atoms occur in several different weights – called isotopes – that vary depending on where the lead originates in the world. The isotopes create “fingerprints” that point to the metal's geographical origin. The scientists in this study determined the ratio of the different types of lead in the regional air samples to identify their telltale fingerprints, and thus, their source.

The lead fingerprints from the two San Francisco sites were compared. The researchers used the ratios of the different lead types to calculate the fraction of lead that came to California from Asia. They compared the fingerprints from the archived air samples from China and California to further test the lead fingerprinting methods and confirm seasonal and distribution findings.

What did they find?

The chemical fingerprints of the Bay Area samples taken in winter showed a similar lead ratio to previous air samples collected from San Francisco and throughout the United States. That is, they had an American fingerprint with relatively high levels of some lead isotopes and low levels of others.

The samples from China, on the other hand, had a different lead fingerprint. The distinctive ratio of lead isotopes matched those identified in prior studies that analyzed air samples from Asia.

Although the winter samples taken in the Bay Area had characteristically American lead fingerprints, samples taken in the spring between March and May – particularly from coastal Mt. Tamalpais – had lead fingerprints similar to the Asian samples. In fact, as much as 80 percent of the lead from the Mt. Tamalpais and 37 percent from the Chabot Observatory in those samples originated in Asia.

The older air samples from California ranged from 0 to 93 percent of lead originating in Asia. Nearly half of the samples had 30 percent, or more, of lead from Asia and the differences appeared to depend on the weather patterns when the samples were taken.

What does it mean?

There is considerable migration of airborne pollution – as measured through a lead marker – from Asia, across the Pacific Ocean and into California. The Asian lead “fingerprint” was pervasive in the coastal and inland California air samples. The amount of lead varied seasonally and was highest in the springtime.

It is known that dust and pollution cross the ocean from the Asian industrial plume. Previous studies show China's industrialization has contributed to rising lead concentrations in the North Pacific Ocean and Canadian ice cores during the last 20 years.

However, this paper adds more precise information about the process and identifies a new tool that can accurately track levels and destination of pollutants. A better understanding of how pollution circles the Earth is necessary because of the impact on climate and human and atmospheric health.

The lead levels measured in this study were low – in the nanogram range. Any health effects from this type of exposure to lead are not known. Still, the lead serves as an efficient marker to map global movement of pollution and shows a constant, yet varied, source of contaminants.

This study focused on migration of pollutants over the Pacific Ocean, but similar scenarios are undoubtedly going on worldwide simply because air currents move globally, crossing international borders. The results highlight the fact that airborne pollution is really an international issue that cannot be tackled exclusively by individual countries.

The airborne lead impacted more than just the coastal areas. Lead pollution from Asia was detected in all of the California sites studied – both in the coastal Bay Area and farther inland. Just how far inland the lead is carried by winds and whether it impacts the entire Pacific coast of North America remains to be determined.

The use of lead “fingerprints” to measure overseas movement of pollutants improves on previous indirect and complicated methods. Unfortunately, it may not be a suitable method to track other air pollutants. Lead, though, may indicate how others – including carbon monoxide, nonmethane hydrocarbons and ground-level ozone – move around the world as pollutants probably travel in similar patterns guided by the air currents.

The problem of imported air pollution is not likely to go away and may actually increase. More emissions and a larger particulate plume will be created as industry expands and new markets emerge in Asia. This study's findings clearly illustrate the potential for these pollutants to be carried long distances. Countries may still be at risk even if they institute policy changes to limit dangerous exposures – such as phasing out leaded gasoline. International cooperation will be needed to truly reduce the environmental and health risks caused by airborne pollutants.


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