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DEVELOPMENT OF PERSONAL OZONE SAMPLERS: THREE APPROACHES
Impact/Purpose:
Ozone is formed in the lower atmosphere when emissions from mobile and industrial sources react in the presence of sunlight. Because ozone is a highly reactive gas and a major component of urban smog, the Environmental Protection Agency has set a standard for ozone of 120 parts per billion for one hour, a level not to be exceeded more than once per year. Ozone is a major public health concern because exercising subjects experience transient decrements in lung function when exposed to ambient air containing ozone at concentrations equal to, or slightly below, the level of the current standard. In addition, because indoor ozone concentrations can exceed 50% of outdoor concentrations, people may be exposed to ozone for several hours each day. Assessing the risk of adverse health effects from such exposures is difficult because only limited data are available on the actual ozone concentrations that people experience.
Fixed-site monitors, which measure ambient ozone levels over widely spaced areas, are large and expensive to operate. Thus, these monitors cannot be used to measure individual exposures of large numbers of human subjects; in addition, their readings may not accurately reflect ozone levels in different microenvironments. In contrast, small personal ozone samplers measure total accumulated exposure as a person moves from one setting to another. Thus, personal samplers can improve the estimate of the exposure that an individual experiences and can provide better information for assessing the risk of exposure to ozone.
The Health Effects Institute supported three studies to advance the development and testing of personal ozone samplers. The results of these studies are discussed breifly here.
Description:
The investigators funded under the HEI ozone sampler program, Drs. Hackney, Yanagisawa, and Koutrakis, and their collaborators used different approaches to develop personal ozone samples that would be sensitive, accurate, and amenable to use in epidemiological studies.
Dr. Hackney and colleagues designed active and passive samplers based on ozone-induced changes in the intensity of a color formed by a chemical reagent. (Passive samplers depend on the natural diffusion of air and gases to the collection site, and active samplers use a pump to draw air into the device.) Dr. Hackney and associates coated filter papers with a reagent that forms a pink color after ozone exposure. By extracting the reaction product from the paper and measuring the intensity of the colored solution, they determined the amount of ozone that had reacted with the reagent. The investigators also designed a light-tight sample holder to mitigate the reagent's sensitivity to light. When tested in an indoor exposure chamber using mixtures of filtered air and pure ozone, the samplers detected ozone relatively accurately at several combinations of temperature and humidity. The active sampler was less accurate at lower humidity than at higher humidity, and the passive sampler's performance declined when it was tested at a combination of high temperature and high humidity. Both active and passive samplers performed less satisfactorily outdoors than indoors. The active device was accurate only for one- to two-hour exposures; the passive device overestimated ozone levels. Based on these results, the investigators concluded that the slower process of air diffusion, inherent in the passive design, allowed an unknown interfering component of ambient air to contact the reagent for a longer time than in the active sampler. For these reasons, the samplers based on the reagent used by Dr. Hackney and colleagues were not considered for further development. Although the reagent appeared promising in early laboratory tests, the investigators' careful experimental work indicated that it was problematic in a practical application.
Dr. Yanagisawa's passive sampler is based on ozone oxidizing iodide ion to molecular iodine. The unique feature of this sampler is that as the reaction proceeds on a carbon disk coated with a nylon derivative and potassium iodide, the volatile iodine product stabilizes by forming an electrically charged complex with nylon. The amount of current discharged by the complex then is a measure of the amount of iodine bound to nylon, and thus, the amount of ozone that had reacted with iodide ion to form molecular iodine. In chamber studies, the amount of charged complex formed increased with increasing ozone exposure levels. The sampler was generally unaffected by wind, temperature, or relative humidity, except at very low humidity levels (12%). Sulfur dioxide decreased iodine formation by ozone, but this interference was eliminated by adding a filter to absorb the sulfur dioxide. However, nitrogen dioxide also was detected, thus interfering with ozone detection. Therefore, Dr. Yanagisawa's sampler must be considered a total oxidant sampler, rather than an ozone-specific sampler.
The third study evaluated the performance of a passive sampler previously designed by Dr. Koutrakis and colleagues. The sampler is based on ozone oxidizing nitrite ion, which is coated onto glass fiber filters, to nitrate ion. After extracting the ions from the filters with water, the investigators separated the nitrate ion from the nitrite ion by a process called ion chromatography. Although it requires a certain technical expertise, this method is efficient and rapid, both positive features when multiple samples from large epidemiological studies must be analyzed. In both indoor chamber experiments and outdoor field tests, good agreement was found between the ozone levels detected by Dr. Koutrakis' sampler and a reference ozone monitor; however, these comparisons need to be made systematically over a wider range of ozone levels than was done in this study. Initial chamber experiments indicated that the sampler was affected by wind velocity. However, wind velocity effects were adequately minimized by the use of a plastic wind shield. The investigators themselves did not perform interference studies as part of their HEI project. Results from another laboratory indicate that nitrogen dioxide, sulfur dioxide, and peroxyacetylnitrate do not interfere significantly with the sampler's performance. The sampler's performance also was not affected by temperature or relative humidity. A major uncertainty that needs to be addressed before the sampler is used in field studies is the method the investigators used to assess the sampler's accuracy. (A similar concern is also relevant to the accuracy of the other samplers discussed in this Research Report.) Sampling rates were determined by an equation that used concentrations derived from a standard reference ozone monitor, and the calculated ozone levels were compared with those obtained with the reference monitor to assess accuracy. Using sampling rate data that are independent of a standard monitor would be a more scientifically rigorous method to estimate an experimental sampler's accuracy. An independent validation of Dr. Koutrakis' sampler would allow researchers to determine if its performance is adequate to meet the objectives of a proposed study.
In summary, HEI-funded investigators examined the performance of ozone samplers based on three different experimental approaches. The binary organic reagent used by Dr. Hackney and colleagues in their active and passive samplers proved unsuitable for accurate colorimetric ozone determination. Dr. Yanagasawa's sampler shows promise, but its ability to detect ozone at low concentrations requires improvement before it is ready for validation studies. If the detection limits can be improved, this sampler may be attractive to many analysts because of its simplicity. At the present time, of the samplers described in this Research Report, Dr. Koutrakis' is the closest to being ready for use in epidemiologic studies.