Air pollution as an underappreciated cause of asthma symptoms.

OR NEARLY 100 YEARS IT HAS BEEN KNOWN THAT asthma is a condition in which an acute respiratory response may follow inhalation of some material to which a person is sensitized. The list of such materials grew slowly over 50 years (it started with horse dandruff), continued with ragweed and grasses, and on the basis of skin responses, was believed to include a wide variety offoodstuffs(includingnuts). 1 Domesticpets,andlater,pests such as cockroaches and house dust mites came to the fore, and as occupational asthma came to be recognized, the list expanded still further. However, the focus of such awareness has been on agents that directly cause allergic reaction, and therefore can be diagnosed via skin tests, rather than on agents that cause nonspecific generalized inflammation, such as air pollution. Recognition that exacerbations of asthma commonly involve inflammation has led to awareness that agents capable of inducing an inflammatory reaction in the lungs can worsen the condition. Early on, air pollution was suspected as one such important inflammatory factor since individuals with asthma were shown to be more sensitive than those without asthma to the gaseous air pollutant sulfur dioxide, 1 and because this type of air pollution was common with uncontrolled coal burning. During the latter part of the 20th century, sulfur dioxide levels were reduced substantially, but nitrogen oxides from power plants have not been as well controlled, while public exposure to motor vehicle emissions, with their complex mixture of particles and gases, has increased. Much of today’s pollution problem is due to “secondary” pollutants, such as gaseous ozone and sulfate particles, from pollutants emitted by power plants, industries, and motor vehicles. Despite the best efforts to date, such secondary air pollution, in the form of ozone and fine particles in the air, has remained at stubbornly high levels in many parts of the United States, in violation of the US Environmental Protection Agency air quality standards. 2 Ozone is a highly reactive gas that results primarily from the action of sunlight on hydrocarbons and nitrogen oxides emitted in fuel combustion. It reacts chemically and “oxidizes” lung tissues on contact, acting as a powerful respiratoryirritantatthelevelsfrequentlyfoundinmostofthe nation’s urban areas during the summer months. 2 Epidemiological and clinical studies have shown that ozone exposure is associated with worsening of athletic performance,reductionsinlungfunction,shortnessofbreath,chest pain with deep inhalation, wheezing and coughing, and asthma exacerbations among those with asthma. 3 Despite this evidence, air pollution remains one of the most underappreciated contributors to asthma exacerbations. In this issue of THE JOURNAL, Gent and colleagues 4 report on their cohort study of asthmatic children from the New Haven, Conn, and Springfield, Mass, areas, including 130childrenwhousedmaintenancemedicationsforasthma and 141 children who did not; the former group was consideredtohavemoresevereasthma.Inthegroupusingmaintenance medication, the level of ozone exposure was significantly associated with worsening of symptoms and an increase in the use of rescue medication. Each 50-ppb increase in 1-hour average ozone level was associated with an increased likelihood of wheezing (by 35%) and chest tightness (by 47%). The findings suggest that asthmatic children who use maintenance medication were particularly vulnerable to ozone, even after controlling for coexposure to fine particles, and even at air pollution levels below current Environmental Protection Agency air quality standards.