Ambient Air Levels Research Articles

Contribution of the intercontinental atmospheric transport to mercury pollution in the Northern Hemisphere

Dispersion of mercury in the atmosphere of the Northern Hemisphere is studied by means of atmospheric modelling. Mercury concentration in the ambient air and deposition levels are investigated as well as the intercontinental transport of mercury is assessed. It is demonstrated that the contribution of external anthropogenic and natural sources to mercury deposition to different continents varies from 30% to 70% of total value. Besides, about half the mercury deposition to such a remote region as the Arctic is due to the transport from anthropogenic emission sources. Thus, it could be concluded that the contribution of the intercontinental atmospheric transport of mercury is comparable with that of regional pollution even in industrially developed regions of the Northern Hemisphere.

Confronting Workplace Exposure to Chemicals with LCA: Examples of Trichloroethylene and Perchloroethylene in Metal Degreasing and Dry Cleaning

Life-Cycle Assessment (LCA) aims to assess all environmental impacts "from cradle to grave". Nevertheless, existing methods for Life-Cycle Impact Assessment (LCIA) generally do not consider impacts from chemical exposure at the workplace. This is a severe drawback, because neglecting occupational health effects may result in product or process optimizations at the expense of workers' health. We adapt an existing LCIA method to consider occupational health effects from the use of perchloroethylene (PCE) and trichloroethylene (TCE) in dry cleaning and metal degreasing. The results show that, in applications such as metal degreasing and dry cleaning, long-term (steady-state) concentrations at the workplace are up to 6 orders of magnitude higher than ambient air levels. Legal threshold values may be exceeded, depending on machine technology, size, and surrounding working conditions. The impact from workplace exposure to the total human-toxicity potential of the complete life cycle of PCE and TCE (including use, production, and disposal) is accordingly high. We therefore conclude that occupational health effects need to be considered in LCA to prevent overlooking key environmental-health impacts in LCA.

Human biomonitoring of cadmium and lead exposure of child–mother pairs from Germany living in the vicinity of industrial sources (Hot Spot Study NRW)

Cadmium (Cd) and lead (Pb) exposure of children and their mothers living in the vicinity of industrial sources (metal refining) was assessed by a cross-sectional study performed in 2000. Study areas were the highly industrialized city of Duisburg and a rural area of North Rhine Westphalia, Germany. Exposure to ambient air concentrations of Cd and Pb was calculated from a Lagrange dispersion model using data sets from ambient air quality measurements. Cd in blood and urine and Pb in blood were measured by AAS. Mean age (years) was 6.4 (range 5.5–7.7) for children ( n = 238 ) and 36 (range 23–48) for mothers ( n = 213 ). A total of 49% of the children were males. Factors suspected to influence metal levels in blood or urine were obtained by questionnaire. Individual ambient Cd and Pb levels according to the home address ranged from 0.5 ng/m 3 (Cd) and 0.03 μg/m 3 (Pb) (rural area) up to 31.2 ng/m 3 (Cd) and 0.73 μg/m 3 (Pb) (industrialized area). Cd levels (geometric mean) in blood (0.13 and 0.10 μg/L) and urine (both areas 0.09 μg/L) of children did not differ between the two areas. Cd levels in blood and urine of mothers from the industrialized area were higher (blood 0.39 μg/L, urine 0.28 μg/L) than in those from the rural area (blood 0.25 μg/L, urine 0.25 μg/L). Pb levels in the blood of children from the industrialized area were higher (31 μg/L) than in those from the rural area (21 μg/L). Pb levels in the blood of mothers did not differ between the two areas (both 24 μg/L). Pb levels in blood showed a significant association between child and mother ( n = 192 ; r = 0.26 , p < 0.001 ). This did not apply for Cd in blood or urine. Regression analysis clearly revealed that Pb levels in ambient air were associated with Pb in the blood of children. Minor associations were also found between Cd in air and Cd in the blood of mothers and between Cd in air and urine of mothers.

Formaldehyde and acetaldehyde associated with the use of natural gas as a fuel for light vehicles

Abstract Data collected from 1998 to 2001 clearly show that formaldehyde levels in ambient air of the city of Rio de Janeiro increased in 2001 (Correa et al., 2003, Atmospheric Environment 37, 23–29). In order to continue this study, samples were collected at the same site in the period from 2001 to 2002. In this work, we present the observed trends for formaldehyde and acetaldehyde levels from 1998 to 2002. Mean formaldehyde levels increased from 20 ppb in 1998 to 80 ppb in 2002, while acetaldehyde concentrations remained nearly unchanged. The formaldehyde/acetaldehyde ratio increased from 1.0 to 4.5 in the same period of time. These results may be explained by the increasing use of compressed natural gas by the vehicular fleet, in substitution of ethanol and gasohol (a mixture of gasoline and ethanol, 24% v/v). In order to confirm this hypothesis, some experiments were carried out to estimate the formaldehyde and acetaldehyde emissions from 20 automobiles powered by natural gas. The results showed a mean formaldehyde/acetaldehyde emission ratio of 3.42 for natural gas-fueled vehicles and of 0.24 when the same vehicles are fueled with gasohol. These high levels of formaldehyde may be attributed to the incomplete combustion of methane (80–90% of the natural gas) that is catalytically converted to formaldehyde in the exhaust pipe.

Development of a Passive Sampler for Long-Term Measurements of Formaldehyde and Total Oxidants in Air

A diffusive sampling device is described that is capable of reliable measurements of formaldehyde and total oxidants (Ox = ozone + nitrogen dioxide) at sub-ppbv concentration levels in ambient air. These species are collected on silica gel particles coated with 1-methyl-1-(2,4-dinitrophenyl)hydrazine (MDNPH) and phosphoric acid. The formaldehyde hydrazone (HCHO-MDNPh) and the N-methyl-2,4-dinitroaniline (MDNA) formed are extracted with acetonitrile and determined by HPLC with UV detection at 365anm. The measured sampling rate for HCHO, 15.0 mLmin-1, agrees well with the theoretical value of 16.0, whilst an experimental sampling rate of 10.7 mLmin-1 (25% lower than the calculated one) is observed for Ox. The sampling rates seem to be independent of the sampling duration up to one month. The precision of the measurements for co-located passive samplers averaged is 7.3% for HCHO and 7.2% for Ox in urban air.

Evaluation of PCDD/F levels in ambient air and soils and estimation of deposition rates in Kocaeli, Turkey

PCDD/F analyses were made in soil and ambient air samples taken from Kocaeli, an industrialized area of Turkey. Results showed that the levels of PCDD/F pollution are comparable to the levels observed in the various urban areas in the world. PCDD/F concentrations in surface soils ranged between 0.4 and 4.27 pg I-TEQ kg −1 with a geometric mean of 0.76 pg I-TEQ kg −1, while those in ambient air were between 23 and 563 fg m −3, with geometric mean of 82.0 fg m −3. Ambient air PCDD/F concentration in the city center was about 10 times higher than those in the rural area. Combustion activities present in the area were assessed to be the most significant source of the PCDD/F pollution based on the congener and homologue profiles and statistical analysis of the data. The deposition rate modeling of USEPA was applied and the deposition rates were determined in the range between 7.6 and 66.3 ng I-TEQ m −2 year −1 with a geometric mean of 15.9 ng I-TEQ m −2 year −1. The rates were higher than the recommended deposition rates in relation to the tolerable daily intake (TDI) range of PCDD/Fs for humans. The deposition velocities computed were also found to be high in both low and highly chlorinated PCDD/Fs, and this was attributed to the vapor phase deposition of volatile PCDD/Fs, and the scavenging effect of the precipitation on the particle-bound PCDD/Fs.

Evaluation of PCDD/Fs patterns emitted from incinerator via direct ambient sampling and indirect serum levels assessment of Taiwanese

The aim of this study was to evaluate the PCDD/Fs patterns in ambient air based on data information emitted from incinerator generated from ambient air measurements and those in serum. Four circular zones, namely A, B, C, and D, were identified based on simulated ambient annual average PCDD/Fs concentrations, from a selected municipal waste incinerator. Sixteen ambient samples were taken from the 4 circular zones across 4-seasons. Eighty-nine volunteers were recruited according to the demographic distribution within each zone. PCDD/Fs profiles were documented both for air and serum samples collected. Comparing to the congener patterns from ambient air and serum samples, we found that OCDD, OCDF, 1,2,3,4,6,7,8-HpCDD, and 1,2,3,4,6,7,8-HpCDF were the predominant groups among 17 congeners from both the ambient air and serum sample. And, factor analysis showed the distribution patterns of PCDD/Fs from ambient air and serum samples are almost identical across different zones, except for congener patterns of serum samples from residents in zone C. In addition, the average PCDD/Fs level significantly reduced for about 10 folds than those of the other three seasons when the incinerator was shut down in one of sampling periods. We might conclude that ambient air exposure was the most important contributor to PCDD/Fs levels in ambient air but not the single in serum. Therefore, another or more powerful source, such as occupational exposure, dietary intake or the consumption of local food, should be further investigated at the same time.

Spatial and Temporal Distribution of Volatile Organic Compounds around an Industrial Park of Taiwan

The results of spatial and temporal distribution of volatile organic compounds (VOCs) in ambient air surrounding the Hsinchu Science-Based Industrial Park (SBIP), Taiwan during February 2001 to November 2001 are presented. The sampling was performed at 13 different sites around the SBIP for 24 hrs once every month, and a total of 130 samples were collected. The effects of geographical and meteorological conditions and production volume of the SBIP on the variation of pollutant’s concentration were investigated. The spatial distribution shows that a comparatively higher concentration of VOCs was found at the sites, which are in the southwestern part of SBIP, downwind of or close to factories. When the wind velocity was low (i.e. below 2 m/s), the concentration of pollutant tended to increase and became uniformly distributed around the science park. The temporal distribution shows a decrease in ambient concentration of pollutants from February to June 2001, mainly due to the decline in the production volume of the SBIP in this period. During the whole period of investigation, the concentration of most of the compounds (except acetone and IPA) was found to be lower than the factory-surrounding air quality standard of Taiwan, but in some cases it was higher than the AALG (ambient air level goal).

Differential optical absorption spectrometer measurement of NO2, SO2, O3, HCHO and aromatic volatile organics in ambient air of Kaohsiung Petroleum Refinery in Taiwan

UV-differential optical absorption spectrometer (DOAS) technique is considered as a promising technique to detect gaseous pollutants and was applied to conduct one-week continuous measurements in the Chinese Petroleum (CPC) refinery plant located in Lin Yuan industrial park of Kaohsiung, Southern Taiwan. With the combination of local meteorological information, including solar radiation, wind direction and speed, the results showed that the concentrations of aromatic compounds and formaldehyde (HCHO) were higher at night while the values of ozone, NO2 and SO2 were high during the day. The major source of aromatics was the aromatic extraction unit in the refinery while NO2 and SO2 were mainly emitted from chimneys with not very high average concentrations. Formaldehyde concentration was above 50 ppbv during night. There exists an apparent correlation between the variation of ground-level ozone concentration and photochemical reactions. The results indicate that in addition to benzene and toluene, ozone is a deleterious pollutant. The commercial DOAS system provides reliable information on distribution patterns of major air pollutants depending on their concentration levels in ambient air.

Can we use fixed ambient air monitors to estimate population long-term exposure to air pollutants? The case of spatial variability in the Genotox ER study

Associations between average total personal exposures to PM2.5, PM10, and NO2 and concomitant outdoor concentrations were assessed within the framework of the Genotox ER study. It was carried out in four French metropolitan areas (Grenoble, Paris, Rouen, and Strasbourg) with the participation, in each site, of 60–90 nonsmoking volunteers composed of two groups of equal size (adults and children) who carried the personal Harvard Chempass multipollutant sampler during 48h along two different seasons (“hot” and “cold”). In each center, volunteers were selected so as to live (home and work/school) in three different urban sectors contrasted in terms of air pollution (one highly exposed to traffic emissions, one influenced by local industrial sources, and a background urban environment). In parallel to personal exposure measurements, a fixed ambient air monitoring station surveyed the same pollutants in each local sector. A linear regression model was accommodated where the dependent pollutant-specific variable was the difference, for each subject, between the average ambient air concentrations over 48h and the personal exposure over the same period. The explanatory variables were the metropolitan areas, the three urban sectors, season, and age group. While average exposures to particles were underestimated by outdoor monitors, in almost all cities, seasons, and age groups, differences were lower for NO2 and, in general, in the other direction. Relationships between average total personal exposures and ambient air levels varied across metropolitan areas and local urban sectors. These results suggest that using ambient air concentrations to assess average exposure of populations, in epidemiological studies of long-term effects or in a risk assessment setting, calls for some caution. Comparison of personal exposures to PM or NO2 with ambient air levels is inherently disturbed by indoor sources and activities patterns. Discrepancies between measurement devices and local and regional sources of pollution may also strongly influence how the ambient air concentrations relate to population exposure. Much attention should be given to the selection of the most appropriate monitoring sites according to the study objectives.

Constraining uncertainties about the sources and magnitude of polycyclic aromatic hydrocarbon (PAH) levels in ambient air: the state of Minnesota as a case study

Emissions data are often lacking or uncertain for many airborne contaminants. Polycyclic aromatic hydrocarbons (PAHs) emitted from combustion sources fall into this category. Currently available ambient-air emission inventories of PAHs either fail to account for population-based activities, such as residential wood combustion (RWC) and motor vehicle (MV) activity, and/or report “total PAH” or particulate organic matter emissions, instead of individual compound emissions. We assess the degree of overlap between predicted concentrations from estimated emissions with measured concentrations. Our analysis is based on probabilistic analysis of measured outdoor air concentrations with those predicted from mass-balance models. Based on available information, we estimate the relative magnitude of emissions from three major sources of PAHs to outdoor air: (1) on-road MVs, including light-duty gasoline vehicles and diesel-powered buses and medium and heavy-duty trucks; (2) RWC; and (3) power generation from external combustion boilers. We use the CalTOX regional multimedia mass-balance model to evaluate our emissions estimates in rural and urban regions of the state of Minnesota, USA. We compare model estimates of outdoor PAH airborne concentrations with those reported by the Minnesota Children's Pesticide Exposure Study (MNCPES). With these measured concentrations we probabilistically evaluate the reliability of our emissions estimates for specific PAHs. The median estimates of our predicted outdoor air concentrations agree within an order of magnitude of measured concentrations. For four representative PAHs, we obtain a reasonable degree of overlap between empirical and predicted distributions of outdoor air concentrations. Our combination of models, emissions estimates, and empirical concentration data estimate exposure in a manner that is more reliable than any of these tools alone. Thereby, we increase our confidence about our plausible ranges of emissions and predicted concentrations.

A chamberless field exposure system for ozone enrichment of short vegetation

Only few studies have been conducted as yet which focus on the effects of rising tropospheric ozone levels on semi-natural vegetation under free-air conditions. A new technical approach was used to examine the response of calcareous grassland to ozone employing a chamberless fumigation system. Four different ozone regimes were applied (1-, 1.33-, 1.66- and 2-fold ambient air levels) with five replicates each. Ozone enrichment was carried out on circular plots of 2 m in diameter by a computer controlled exposure system. Transparent windscreens encircling each plot accelerated the mixing of ambient air and ozone released. Thus, the use of blowers could be avoided. The exposure system presented here is regarded as an appropriate technique for free-air trace gas enrichment on short vegetation avoiding microclimatic alterations known to affect plant growth and pollutant uptake. Furthermore, the chosen technical set-up was rather cost-effective. Hence, it enabled the establishment of a larger number of replications providing the basis for results of higher statistical power.

Temporal trends of PCDDs/PCDFs in ambient air in Catalonia (Spain)

This paper reports the results of an assessment of dioxin levels in ambient air in samples collected in the four provinces of Catalonia (Spain) performed by the Environment Department of the Autonomous Government of Catalonia and the Spanish Council for Scientific Research (CSIC). The study includes compiled data of more than 133 samples collected in 28 different sites (rural, urban, suburban and industrial) between 1994 and 2002. The levels revealed a variable content of polychlorinated dibenzo- p-dioxins/polychlorinated dibenzofurans (PCDDs/PCDFs) depending both on the area and the contamination source. Thus, concentrations from 16 to 954 fg I-TEQ/Nm 3, with a mean value of 180 fg I-TEQ/Nm 3, were determined in industrial areas. The levels found in urban and suburban sites varied from 10 to 357 fg I-TEQ/Nm 3, with a mean value of 80 fg I-TEQ/Nm 3. The lowest concentrations were found in rural areas, ranging from 5 to 125 fg I-TEQ/Nm 3, with a mean value of 42 fg I-TEQ/Nm 3. As part of the project, levels of samples collected in parallel using two different samplers, a total suspended particulate (TSP) sampler and PM10 sampler, were compared. The results of 11 different campaigns indicated that both methods are comparable and no significant differences were determined.

Air‐surface exchange of peroxyacetyl nitrate at a grassland site

Direct measurements of the dry deposition velocity of peroxyacetyl nitrate (PAN) were made during the daytime between the months of July and October above a grassland surface in northern Illinois by a modified Bowen ratio technique. Differences in the air temperature, water vapor content, and PAN concentration were measured between the heights of 3.0 m and 0.92 m. Although the measurement uncertainties were large, the cumulative data indicate a slight downward flux of PAN, with an average and standard error of 0.13 ± 0.13 cm s−1 for the dry deposition velocity. Theoretical calculations showed that thermochemical decomposition of PAN on leaf and soil surfaces heated to temperatures above the ambient air levels would contribute less than 15% of the total PAN flux at the elevations of the PAN measurements. A theoretical evaluation of the transfer of PAN through leaf stomata and the plant cuticular membrane indicated that uptake of PAN by vegetation during the daytime is controlled by transfer through the leaf stomata rather than the cuticular membrane. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O3. The mesophyll resistance for O3 is also expected to be less than the value for PAN, because O3 has more reaction sites within plant cells and reacts faster than PAN with protein thiols of the cell membranes. Measurements from other studies indicate that the dry deposition velocity for PAN above a vegetated surface during the daytime is lower by a factor of 0.5–0.3 than for O3. Our measurements of the PAN deposition velocity agree with the results of previous studies and with theoretical calculations based on the physicochemical properties of PAN and the grassland surface. These measurements imply that removal of PAN from the daytime atmospheric boundary layer by thermochemical decomposition is more rapid than dry deposition to a grassland surface.

Dermal exposure assessment to benzene and toluene using charcoal cloth pads

Charcoal cloth pads have been used to assess volatile chemicals on the skin in a laboratory setting; however, they have not yet been applied to measure dermal exposure in occupational settings. This study aimed at evaluating whether charcoal pads can be used to assess dermal exposure to benzene and toluene in workers of a petrochemical plant. Inhalation and dermal exposure levels to benzene and toluene were assessed for workers of a petrochemical plant performing different jobs. Benzene uptake was assessed by determining S-phenylmercapturic acid in workers' urine samples. Dermal exposure levels on the charcoal pads were adjusted for ambient air levels of benzene and toluene by subtracting the amount of benzene or toluene measured in personal air from the amount of benzene or toluene measured on the charcoal pad. In general, measured external and internal exposure levels were low. The estimated contribution of the dermal route to internal benzene exposure levels was less than 0.06% for all jobs. Toluene personal air concentrations and benzene and toluene dermal exposure levels differed statistically significantly between job titles. For benzene, differences between jobs were larger for adjusted dermal exposures (maximum 17-fold, P = 0.02) than for inhalation exposures (maximum two-fold, P = 0.08). Also for toluene, although less clear, differences between jobs were larger for adjusted dermal exposures (maximum 23-fold, P = 0.01) as compared to inhalation exposures (maximum 10-fold, P = 0.01). Charcoal pads appeared to measure dermal exposures to benzene and toluene in addition to ambient air levels. Future studies applying charcoal cloth pads for the dermal exposure assessment at workplaces with higher dermal exposure to organic solvents may provide more insight into the biological relevance of dermal exposure levels measured by charcoal cloth pads. In addition, the design of the dermal sampler might be improved by configuring a dermal sampler, where part of the sampler is protected against direct contact and splashes, but still permeable for the gas phase. This design would most likely result in a better ability to correct for airborne concentrations at a given body location.

Comparison of BTX Measurements Using a Differential Optical Absorption Spectroscopy and an On-Line Gas Chromatography System

The suitability of the DOAS system for the analysis of aromatic VOCs including benzene, toluene, and xylenes in air were tested against an on-line GC measurement technique. Results of our study indicate that the compatibility between two different methodologies, when checked in terms of various standards, is highly variable depending on species. If the differences in measured values between the two are calculated in terms of percent differences, the level of agreement is found to decrease on the order of toluene (-3.4%), xylenes (-45.5%), and benzene (-83.0%). The results of linear regression analysis indicate that the correspondence between the two methods can be expressed as follows: Benzene (DOAS) = 0.240 Benzene (GC) + 0.514 (r = 0.128) Toluene (DOAS) = 0.857 Toluene (GC) + 0.692 (r = 0.711) Xylenes (DOAS) = 0.771 Xylenes (GC) + 0.269 (r = 0.647) A comparison of the data sets obtained by the two distinctive systems reveals that toluene maintains the strongest compatibility between the two systems, while benzene tends to record the least agreement. Although the system-to-system bias is different among different chemical species, the overall results of our study suggest that the use of the commercial DOAS system can provide reliable information of temporal and spatial distribution patterns of major aromatic VOCs, depending on their concentration levels in ambient air (e.g., well above their detection limits).

Atmospheric Formaldehyde Monitoring in the Greater Houston Area in 2002

A laser spectrometer based on difference frequency generation (DFG) was deployed for real-time long-term monitoring of HCHO concentrations at an environmental monitoring site located at Deer Park, Texas, in the Greater Houston area. Three HCHO concentration measurements were made during the periods of July 20-31 (period I), August 2-14 (period II), and August 24-September 25 (period III), 2002. In periods I and II, differences in HCHO concentrations are apparent between day and night measurements, with elevated concentrations during daylight hours. Most of the HCHO peak values are less than 20 ppbV except for two intense peaks on August 02 (approximately 25 ppbV) and August 04 (approximately 30 ppbV). The formaldehyde concentration levels in ambient air at the measurement site are produced mainly by the photochemical oxidation of volatile organic compounds (VOCs) caused by intense sunlight during periods I and II. This observation was made based on a comparison with the ozone concentration, solar radiation, temperature, relative humidity, and wind speed data obtained from the Texas Commission on Environmental Quality (TCEQ). During period III, data collected by a time-integrating wet-chemical technique are compared to the data collected by the spectroscopic instrument.

The benzene problem: impact of three-way catalyst technology: potential for further improvement

Chemical ionisation mass spectrometry (CI-MS) was applied to study the benzene emission characteristics of a TWC-vehicle at a time resolution of one Hertz. Three important operating conditions with increased emissions were identified: at vehicle start; at extended stop-and-go situations; and whenever a catalyst-induced benzene formation occurs. The cold start influence was detectable for about 200 seconds of driving corresponding to a distance of 1.2 km. At hot engine/catalyst mean pre- and post-catalyst emission rates of 25–150 mg km-1 and 0.1–135 mg km-1 were determined. Catalyst conversion varied from 0.07 to >0.99. Even negative conversion efficiencies were observed at several occasions, indicating that benzene can be formed de novo in a TWC. It is of importance to lower benzene emissions at these critical operating conditions to further reduce ambient air levels in cities and with it the cancer risk for large proportions of our population.

Lead Levels in Ambient Air and Blood of Pregnant Mothers from the General Population of Lucknow (U.P.), India

Lead Levels in Ambient Air and Blood of Pregnant Mothers from the General Population of Lucknow (U.P.), India

Indoor health: background levels of fungi.

There is no uniformity in the suggested guidelines for acceptable levels of fungi in indoor ambient air. Thus, health professionals have no way to determine what levels of fungi may pose a threat to human health. The authors reviewed the published literature to identify data reported for noncomplaint structures, that is, structures in which occupants did not have health concerns associated with the quality of the indoor air. For both commercial and residential structures, fungal concentrations detected were often higher than currently suggested guidance values. The average indoor air concentration in 149 noncomplaint commercial buildings was 233 colony forming units (CFU) per cubic meter, whereas outdoor ambient air levels averaged 983 CFU/m(3). Total indoor spore counts ranged from 610 to 1040 spores/m(3) in three commercial buildings. Outdoor total spore counts associated with these buildings ranged from 400 to 80,000 spores/m(3). The average indoor concentration reported for 820 noncomplaint residential structures was 1252 CFU/m(3) with an average outdoor level of 1524 CFU/m(3). Total spore counts detected indoors at 85 residential structures ranged from 68 to 2307 spores/m(3). Outdoor spore levels associated with these structures ranged from 400 to 80,000 spores/m(3). A large proportion of both commercial and residential noncomplaint structures have indoor ambient air fungal concentrations above 500 CFU/m(3), a level often advocated as requiring remediation in structures when occupants complain of nonspecific adverse health symptoms.