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Description : Naphthalene is rather a special compound in terms of its properties and chemical structure. It is a flammable white solid with the formula C10H8 and the structure of two fused benzene rings, with melting and boiling points of 80.5 and 218 °C, respectively. Its odor is fairly distinctive but not unpleasant, and its odor threshold is about 440 µg m-3 . It is classified as a semi-volatile organic compound (SVOC) due to its vapor pressure of 0.087 mmHg at 25 °C, which is just below the 0.1 mmHg cut-off often used to define volatile organic compounds (VOCs).

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Naphthalene Sources and Exposures Relevant to Indoor and Outdoor Air Chunrong Jia and Stuart Batterman 20 July 2010
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CONTENTS Introduction Methods Result Conclusion Reference
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01 INTRODUCTION
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Naphthalene is rather a special compound in terms of its properties and chemical structure. It is a flammable white solid with the formula C10H8 and the structure of two fused benzene rings, with melting and boiling points of 80.5 and 218 °C, respectively. Its odor is fairly distinctive but not unpleasant, and its odor threshold is about 440 μg m-3 . It is classified as a semi-volatile organic compound (SVOC) due to its vapor pressure of 0.087 mmHg at 25 °C, which is just below the 0.1 mmHg cut-off often used to define volatile organic compounds (VOCs) .
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Naphthalene is a toxic air pollutant widely found in ambient and indoor air due to emissions from the chemical and primary metals industries, biomass burning, gasoline and oil combustion, tobacco smoking, the use of mothballs, fumigants and deodorizers, and many other sources. Naphthalene is widely used as an intermediate in the production of phthalic anhydride (66,000 metric tons in the U.S. in 2000), surfactants (27,000 tons) and pesticides (14,000 tons) . It is also found in many other environments, e.g., 40% of National Priority List (Superfund) sites contain naphthalene in soils.. Naphthalene is also called tar camphor, naphthene, naphthalin, naphthaline, mothballs, mothflakes and white tar; trade names include albocarbon and dezodorator.
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However, naphthalene sublimes rapidly at room temperatures. Due to its bicyclic aromatic structure, naphthalene is also a polycyclic aromatic hydrocarbon (PAH), and it is the most volatile member of this group. Naphthalene has been a target compound in environmental studies examining VOCs, SVOCs and PAHs. While known as a common and widespread air contaminant for many years , naphthalene received relatively little attention prior to the finding of its carcinogenicity in rats in 2000 . polycyclic aromatic hydrocarbon (PAH) Semivolatile organic compounds (SVOCs) Volatile Organic Compounds (VOCs)
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The general public is exposed to naphthalene mainly through inhalation of ambient and indoor air, followed by dietary and non-dietary ingestion . However, no study has explicitly compared exposures across multiple routes for the general population. One estimate of the average intake rate for inhalation is 19 μg day-1, and 0.002 to 4.0 μg day-1 for ingestion of water . For nonsmokers exposed to environmental tobacco smoke (ETS) in their residences, the intake rate due to residential ETS is 1 to 3 μg day-1 . High exposures can occur among workers in industries where naphthalene is present at high concentrations, e.g., mothball manufacturing and creosote-impregnation facilities . High exposures also have been suggested among forest firefighters .
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This paper reviews the more recent naphthalene literature, focusing on sources and concentrations in residential indoor environments and outdoor air. We derive representative estimates of naphthalene concentrations that can be used to estimate background levels and risks, and we discuss determinants of exposure, information intended to inform the development of policies and regulations aimed at improving air quality and reducing exposures. Our scope largely excludes occupational and industrial settings, though we note that many workers may be exposed, e.g., in an older study, NIOSH estimated that 112,700 workers were potentially exposed to naphthalene. Exposure concentrations from 0.7 to 93.2 μg m-3 have been reported in industrial settings in Germany , and have exceeded 1,000 μg m-3 in aluminum, coke, creosote and iron industries in Europe , much higher than seen in the outdoor, indoor and personal air studies discussed in the body of this paper.
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Methods 02
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We searched the scientific literature dealing with sources, concentrations and/or exposure measurements of naphthalene in residences and ambient air. Five major online databases were used: Science Citation Index Expanded (1900-present, ISI Web of Knowledge), which contains peerreviewed journal articles; Conference Proceedings Citation Index - Science (ISI Web of Knowledge), which searches in many conferences, symposia, seminars, colloquia, workshops, and conventions in a wide range of disciplines; ScienceDirect, which contains recent and but still in press peer-reviewed journal articles, many of which are not searchable in the ISI listings; Medline CSA), which focuses on biomedical literature but includes medical, epidemiological and exposure studies reporting on chemical exposures; ProQuest, which includes dissertations and theses that may not be published in the journal literature. Literature sources
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We focused on measures of central tendency (medians and means) and extreme values (90th and 95th percentiles, maximum). Given a lognormal distribution, a common assumption in air pollution studies , the geometric mean (GM) can be derived as: GM=AM/√1+CV2 AM = arithmetic mean CV = coefficient of variation The median are approximately equal under a log-normality assumption, the median was approximated using the GM if the median was not reported. Data analysis
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RESULT 03
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3.1 Emission sources of ambient and indoor naphthalene Emission source of naphthalene may be classified in several ways. Based on the generation mechanism, naphthalene is emitted as a product of incomplete combustion, e.g., from wood, straw, tobacco, gasoline and diesel combustion, and from evaporation or sublimation of naphthalene containing materials, e.g., coal tar, crude oil, petroleum products, moth repellents and air fresheners. Emissions can be classified as natural sources, e.g., wildfires, and as anthropogenic sources, which are a much larger and more significant source of exposure. From an emission inventory perspective, it is useful and common to classify sources as industrial, mobile, agricultural, natural and domestic.
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Environmental releases of naphthalene reported under the U.S. Toxics Release Inventory (TRI) in 2002 were 2.07 million pounds to air (mostly from combustion) and 0.37 million pounds to land ; this inventory emphasizes industrial sources. Considerably larger emissions have been reported due to naphthalene’s use as a moth repellent is 5,500 metric tons were estimated to have been released in 1989 from this use ; a more recent estimate of naphthalene’s use as a pesticide is 3,400 metric tons.
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3.2 Combustion sources The formation of naphthalene (and other PAHs) during combustion has been extensively studied. Combustion is considered to be the single largest emission source of naphthalene . Fuel type, oxygen supply and temperature are the major factors affecting emissions . Naphthalene formed during combustion is predominantly in the vapor phase , e.g., 90% was measured from burning of rice and bean straw .
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3.3 Exposure concentrations In the atmosphere, naphthalene exists in both vapor and particulate phases. The distribution between these phases depends on temperature, precipitation and other environmental factors, but most naphthalene (>95%) occurs in the vapor phase . Naphthalene’s atmospheric lifetime is short, less than 1 day. The dominant transformation process is reaction with photochemically produced hydroxyl and nitrate radicals, which produces nitro-naphthalene as a major product . Rates of these reactions, and the atmospheric lifetime, will depend on ambient temperatures, the amount of sunlight, and the mix and concentration of other atmospheric constituents.
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3.4 Indoor concentrations Regional differences. Figure 1 displays ranges of median and mean concentrations for the U.S., Canadian and European studies. Naphthalene concentrations in US and Canada residences are similar. The North American studies show the widest range of concentrations, and levels generally exceed those in European studies, which show median concentrations below 0.6 μg m3..
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3.4.1 Urban vs. rural areas. Indoor concentrations did not show significant differences between rural and urban residences in one study . Other (inter-study) comparisons also suggest little if any difference, e.g., the median concentration in a rural area in Missoula, MT, USA was only 0.3 μg m-3 , while similar or lower concentrations have been found in urban settings, e.g., 0.18 μg m-3 in the Chicago area . Thus, excluding the measurements in rural areas from the derivation of representative concentration ranges does not significantly affect the indoor concentration ranges.
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3.4.2 Long-term trends. Indoor levels did not show any clear trend, even after removing observations from smoking and complaint residences, or after stratifying by region or country. This differs from the decreasing trend seen for many indoor VOCs in North America and Europe , and it is somewhat surprising given naphthalene's decreasing share of the moth repellent market. The flat trend may hint that the older studies did not fully characterize naphthalene levels, possibly due to measurement problems as discussed later.
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3.5 Outdoor concentrations Maximum outdoor concentrations ranged from 0.01 to 4.7 μg m-3 . The upper bound concentration, about 5 μg m-3, might be used as a conservative exposure scenario for “general” urban air. Occasionally, higher concentrations have been detected, 25 μg m-3 was observed outside of eight Hangzhou, China homes , but this may have resulted from entraining indoor air. Levels may be elevated near industrial and waste disposal sites containing strong sources, e.g., naphthalene levels averaged 11 μg m-3 at a landfill site in summer in Guangzhou, South China. Outdoor concentrations also show seasonal variations, and higher levels are seen in winter as compared to summer . Seasonal changes were 0.96 μg m-3 in Los Angles and 0.20μg·m-3 in Riverside, California, USA . In general, the seasonal changes follow patterns seen for VOCs, i.e., in winter, cool temperatures are often associated with decreased photochemical reaction rates , increased emissions from heating sources , and decreased dispersion due to more stable air and lower mixing heights.
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3.5.1 Regional differences. Median outdoor concentrations of naphthalene in urban areas ranked by region generally follow the following trend: U.S.A. > Europe > Canada . Median concentrations measured elsewhere fall within the bounds of these studies. It is not feasible to compare ambient concentrations measured in rural or remote areas due to the very low concentrations seen and the small number of studies.
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3.6 Health risk assessment The estimated cancer risks from naphthalene exposure are notable. The 2002 National-Scale Assessment concluded that naphthalene in ambient air was a regional cancer risk “driver”, defined as an air toxic where the typical individual chronic cancer risk exceeded 10-5 . Naphthalene was ranked the second highest indoor risk driver and the third highest outdoor risk driver in a Michigan study . Loh et al. derived personal exposures using a microenvironmental model, and ranked naphthalene as ninth among 19 carcinogenic air pollutants. Using the draft URE, cancer risks will increase 3-fold and typical risk levels will approach or fall into the 10-4 range, while peak measurements, if reflective of chronic exposure, represent cancer risks in the 10-3 or possibly even higher range. If the tumor response in experimental animals is confirmed to be predictive of human health risk, these risks will be significant, especially given the millions of individuals exposed.
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DISCUSSION 04
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Naphthalene is one of the least volatile VOCs and the most volatile PAH. Because this compound has often been excluded in both VOC and PAH studies, the exposure-relevant literature is, in many ways, deficient and inferior to the VOC and PAH literature. Still, we identified 20 recent indoor studies and 21 recent outdoor studies, which were used to derive representative ranges of concentrations applicable to residences and urban settings. Only three studies making personal measurements in community settings were identified, all in Europe.
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Conclusion
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Concern regarding human exposure to naphthalene through inhalation has greatly increased due to its potential carcinogenicity, which was discovered in 2000 . We derived representative ranges of residential, outdoor and personal concentrations of naphthalene, emphasizing the more recent literature. This literature is limited, especially for personal exposures. Considering what is available, we conclude that personal and residential concentrations are similar, while ambient concentrations are about an order of magnitude lower. Our estimate of representative ranges of indoor concentrations are about 0.2 to 2 μg m-3 for medians, and about twice that for averages.
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There is a need to validate and intercompare VOC and PAH measurement techniques. This will also ensure the comparability of studies and reduce uncertainties. The spatial and temporal variability of concentrations near roads, industrial and other sources, is poorly characterized. Factors affecting indoor concentrations, including the causes of the highest levels, are not well understood, and populations at risk of high of exposure presently cannot be identified. Better information regarding product usage patterns, emission rates of consumer products, building materials, and other sources of naphthalene is needed, as are long term measurements. Future studies might address losses due to adsorption onto building materials, chemical reactions, and utilize multicompartment models to better understand current and estimate historical exposures.
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Reference www.researchgate.net www.ncbi.nlm.nih.gov

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