New Medical Study Says Diesel Exhaust May Cause Asthma, Not Just Aggravate It

NRDC and UCSF Report Shows Strongest Evidence to Date on Causal Link Between Diesel and Asthma

SAN FRANCISCO (February 13, 2002) -- Diesel exhaust doesn't just aggravate asthma, it may cause asthma, according to a new study by researchers from the Natural Resources Defense Council (NRDC) and the University of California at San Francisco (UCSF) School of Medicine. The study in the journal Environmental Health Perspectives suggests that fine particulates found in diesel exhaust lead to immune system alterations that can cause asthma in otherwise healthy people. Asthma is a chronic disease characterized by inflammation of the airways and lungs and is considered an epidemic in the United States.

Diesel exhaust from buses, trucks, other vehicles and equipment is a major source of sooty particles in the air. "There are a lot of sick people at the end of the tailpipe, especially children," said Dr. Gina Solomon, M.D., NRDC senior scientist and co-author of the study. "The tiny particles in diesel exhaust bypass our respiratory defenses and lodge deep into the lungs. Once there, they stimulate an immune response that triggers inflammatory changes, airway constriction, mucus production and symptoms of asthma."

Diesel exhaust also contains nitrogen oxides (NOx), a major component of smog. Although smog has been linked to asthma before, this study draws the strongest link to date between asthma and the small particles in diesel exhaust.

The study summarized several immune pathways that are affected by diesel particles. The particles alter the ratios of certain key immune cells known as T-lymphocytes. Scientists believe these cells play a particularly important role in airway inflammation and allergic response. Diesel exhaust appears to stimulate production of a kind of T-lymphocyte commonly found in people who suffer from asthma and other allergic disease.

Diesel particles also stimulate the production of chemical messengers, known as cytokines and chemokines. These chemicals stimulate inflammation and increase the presence of immune cells that release free radical molecules, which damage the cells in airways. In addition, these chemicals directly stimulate production of mucus, cause constriction of smooth muscles in the airways, irritate the nerves (resulting in coughing), and cause fluid to leak into the airways.

By clarifying the entire pathway of the immune response, the study's authors say they have shown how diesel can work both acutely to trigger attacks and chronically to make an individual susceptible to developing an asthmatic response. Possibly because of this dual effect, individuals exposed to diesel exhaust and a common allergen, such as pollen, will have a greatly magnified response to the allergen. This mechanism may also help explain why more and more children appear to be developing sensitivity to pollen, animal dander, dust mites, and other natural allergens that have been around for centuries.

Diesel School Buses Put Children at Increased Risk

The study adds to the growing catalog of research implicating diesel in a host of ailments, from cancer and cardiac problems to asthma and other respiratory disease. It also underscores the particular danger to children, whose immune systems and lungs are underdeveloped and more vulnerable to pollutants. Pediatric asthma in the United States has increased by 58 percent since 1982.

According to the Centers for Disease Control and Prevention, the number of individuals with asthma increased by 42 percent in the United States during the last decade. If rates continue unchecked, a child born a generation from now will be twice as likely to develop asthma as a child born today. The Pew Environmental Health Commission has calculated that 22 million Americans will suffer from asthma by the end of this decade, 8 million more than today. That means that 1 in 14 Americans and 1 in every 5 families would be forced to live with the disease. By 2020, the commission estimates the number of sufferers could increase to 29 million -- more than twice the current number. In 1990, asthma cost society $6.2 billion. That figure is $11 billion today and could rise to $18 billion by 2020.

School buses are a major culprit of diesel exhaust exposure in children. Not only do children breathe air polluted by buses idling at bus stops and schoolyards, but they also are exposed to diesel exhaust inside the buses themselves. Last year NRDC released, No Breathing in the Aisles: Diesel Exhaust Inside School Buses, a study which showed that diesel exhaust levels on buses were more than eight times the average levels found in the ambient air in California and 23 to 46 times higher than levels considered to be a significant cancer risk.

The article, "Diesel Exhaust and Asthma: Hypotheses and Molecular Mechanisms of Action," appeared in the peer reviewed journal, "Environmental Health Perspectives," Volume 110, Supplement 1, February 2002.

The Natural Resources Defense Council is a national, non-profit organization of scientists, lawyers and environmental specialists dedicated to protecting public health and the environment. Founded in 1970, NRDC has more than 500,000 members nationwide, served from offices in New York, Washington, Los Angeles and San Francisco.

Related NRDC Pages

Diesel Exhaust and Asthma: Hypotheses and Molecular Mechanisms of Action (PDF, 575k)

No Breathing in the Aisles: Diesel Exhaust Inside School Buses