Reply to Viner, A.; Ayrey, S. Comment on "Scheepers et al. Comparative Performance Testing of Respirator versus Surgical Mask Using a Water Droplet Spray Model. Int. J. Environ. Res. Public Health 2021, 18, 1599".

The response to our publication by Andrew Viner and Stewart Ayrey is much appreciated [...].

Determination of cytotoxicity following oxidative treatment of pharmaceutical residues in wastewater.

Pharmaceutical residues are released in the aquatic environment due to incomplete removal from wastewater. With the presence of multiple chemicals in sewage waters, contaminants may adversely affect the effectiveness of a wastewater treatment plant (WWTP). In certain cases, discharged metabolites are transformed back into their pristine structure and become bioactive again. Other compounds are persistent and can withstand conventional wastewater treatment. When WWTP effluents are released in surface waters, pristine and persistent chemicals can affect the aquatic environment. To complement WWTPs and circumvent incomplete removal of unwanted chemicals or pharmaceuticals, on-site wastewater treatment can contribute to their removal. Advanced oxidation processes (AOPs) are very powerful techniques for the abatement of pharmaceuticals, however, under certain circumstances reactive toxic by-products can be produced. We studied the application of on-site AOPs in a laboratory setting. It is expected that treatment at the contamination source can eliminate the worst polluters. Thermal plasma and UV/H2O2 oxidation were applied on simulation matrices, Milli-Q and synthetic sewage water spiked with 10 different pharmaceuticals in a range of 0.1 up to 2400 µg/L. In addition, untreated end-of-pipe hospital effluent was also subjected to oxidative treatment. The matrices were activated for 180 min and added to cultured HeLa cells. The cells were 24 h and 48 h exposed at 37 °C and subsequently markers for oxidative stress and viability were measured. During the UV/H2O2 treatment periods no toxicity was observed. After thermal plasma activation of Milli-Q water (150 and 180 min) toxicity was observed. Direct application of thermal plasma treatment in hospital sewage water caused elimination of toxic substances. The low cytotoxicity of treated pharmaceutical residues is likely to become negligible if plasma pre-treated on-site wastewater is further diluted with other sewage water streams, before reaching the WWTP. Our study suggests that AOPs may be promising technologies to remove a substantial portion of pharmaceutical components by degradation at the source. Further studies will have to be performed to verify the feasibility of upscaling this technology from the benchtop to practice.

Thermal plasma activation and UV/H2O2 oxidative degradation of pharmaceutical residues.

The aquatic environment becomes increasingly contaminated by anthropogenic pollutants such as pharmaceutical residues. Due to poor biodegradation and continuous discharge of persistent compounds in sewage water samples, pharmaceutical residues might end up in surface waters when not removed. To minimize this pollution, onsite wastewater treatment techniques might complement conventional waste water treatment plants (WWTPs). Advanced oxidation processes are useful techniques, since reactive oxygen species (ROS) are used for the degradation of unwanted medicine residues. In this paper we have studied the advanced oxidation in a controlled laboratory setting using thermal plasma and UV/H2O2 treatment. Five different matrices, Milli-Q water, tap water, synthetic urine, diluted urine and synthetic sewage water were spiked with 14 pharmaceuticals with a concentration of 5 µg/L. All compounds were reduced or completely decomposed by both 150 W thermal plasma and UV/H2O2 treatment. Additionally, also hospital sewage water was tested. First the concentrations of 10 pharmaceutical residues were determined by liquid chromatography mass spectrometry (LC-MS/MS). The pharmaceutical concentration ranged from 0.08 up to 2400 µg/L. With the application of 150 W thermal plasma or UV/H2O2, it was found that overall pharmaceutical degradation in hospital sewage water were nearly equivalent to the results obtained in the synthetic sewage water. However, based on the chemical abatement kinetics it was demonstrated that the degree of degradation decreases with increasing matrix complexity. Since reactive oxygen and nitrogen species (RONS) are continuously produced, thermal plasma treatment has the advantage over UV/H2O2 treatment.

Comparative Performance Testing of Respirator versus Surgical Mask Using a Water Droplet Spray Model.

BACKGROUND: During the SARS-CoV-2 pandemic, there was shortage of the standard respiratory protective equipment (RPE). The aim of this study was to develop a procedure to test the performance of alternative RPEs used in the care of COVID-19 patients. METHODS: A laboratory-based test was developed to compare RPEs by total inward leakage (TIL). We used a crossflow nebulizer to produce a jet spray of 1-100 µm water droplets with a fluorescent marker. The RPEs were placed on a dummy head and sprayed at distances of 30 and 60 cm. The outcome was determined as the recovery of the fluorescent marker on a membrane filter placed on the mouth of the dummy head. RESULTS: At 30 cm, a type IIR surgical mask gave a 17.7% lower TIL compared with an FFP2 respirator. At 60 cm, this difference was similar, with a 21.7% lower TIL for the surgical mask compared to the respirator. When adding a face shield, the TIL at 30 cm was further reduced by 9.5% for the respirator and 16.6% in the case of the surgical mask. CONCLUSIONS: A safe, fast and very sensitive test method was developed to assess the effectiveness of RPE by comparison under controlled conditions.

Assessment of exposure of gas station attendants in Sri Lanka to benzene, toluene and xylenes.

Exposure to benzene, toluene and p-, m-, o-xylene (BTX) was studied in 29 gas station attendants and 16 office workers in Sri Lanka. The aim of this study was to assess the exposure level and identify potential exposure mitigating measures. Pre- and post-shift samples of end-exhaled air were collected and analysed for BTX on a thermal desorption gas chromatography mass spectrometry system (TD-GC-MS). Urine was collected at the same timepoints and analysed for a metabolite of benzene, S-phenyl mercapturic acid (SPMA), using liquid chromatography-mass spectrometry (LC-MS). Environmental exposure was measured by personal air sampling and analysed by gas chromatography flame ionization detection (GC-FID). Median (range) breathing zone air concentrations were 609 (65.1-1960) µg/m3 for benzene and 746 (<5.0-2770) µg/m3 for toluene. Taking into account long working hours, 28% of the measured exposures exceeded the ACGIH threshold limit value (TLV) for an 8-h time-weighted average of 1.6 mg/m3 for benzene. Xylene isomers were not detected. End-exhaled air concentrations were significantly increased for gas station attendants compared to office workers (p < 0.005). The difference was 1-3-fold in pre-shift and 2-5-fold in post-shift samples. The increase from pre-to post-shift amounted to 5-15-fold (p < 0.005). Pre-shift BTX concentrations in end-exhaled air were higher in smokers compared to non-smokers (p < 0.01). Exposure due to self-reported fuel spills was related to enhanced exhaled BTX (p < 0.05). The same was found for sleeping at the location of the gas station between two work-shifts. Benzene in end-exhaled air was moderately associated with benzene in the breathing zone (r = 0.422; p < 0.001). Median creatinine-corrected S-phenyl mercapturic acid (SPMA) was similar in pre- and post-shift (2.40 and 3.02 µg/g) in gas station attendants but increased in office workers (from 0.55 to 1.07 µg/g). In conclusion, working as a gas station attendant leads to inhalation exposure and occasional skin exposure to BTX. Smoking was identified as the most important co-exposure. Besides taking preventive measure to reduce exposure, the reduction of working hours to 40 h per week is expected to decrease benzene levels below the current TLV.

Intrusion of chlorinated hydrocarbons and their degradation products from contaminated soil. Measurement of indoor air quality and biomonitoring by analysis of end-exhaled air.

A historic soil contamination of chlorinated hydrocarbons from a former dry cleaning shop caused intrusion of vapors into a building currently used as bookshop. The aim of this study was to determine the indoor air quality and the uptake of soil contaminants and their degradation products. Samples of indoor air were collected over one week in the warm and one week in the cold season. Pre-shift and post-shift samples of end-exhaled air were collected from two employees. Chlorinated hydrocarbons were analyzed in indoor air and exhaled air samples using thermal desorption gas chromatography mass spectrometry (TD-GC-MS). Tetrachloroethylene (PER), and its degradation products trichloroethylene (TRI), 1,1-dichloroethylene (1,1-DCE), 1,2-cis-dichloroethylene (1,2-cis-DCE), 1,2-trans-dichloroethylene (1,2-trans-DCE), methylene chloride (MC) and vinyl chloride (VC) were determined in ambient air. PER was the prime contaminant with a week average (±sd) of 805.2 ±â€¯598.6 µg/m3 in June 2016 and 1031 ±â€¯499.3 µg/m3 in December 2017. MC, 1,2-cis-DCE and TRI were detected at concentrations below 2.3 µg/m3. 1,1-DCE and VC were not detected. In exhaled air PER, 1,1-DCE, and MC were detected in both June and December, whereas TRI, 1,2-cis-DCE and 1,2-trans-DCE were only detected on one or two days in the cold season. VC was not detected in exhaled air. For PER, the mean concentrations (±sd) in end-exhaled air increased from a five days (Mon-Fri) average pre-shift value of 22.2 ±â€¯8.0 to a post-shift value of 52.6 ±â€¯15.5 ng/L in the male shop owner (p < 0.01) and in the female cashier these values were 26.0 ±â€¯3.6 and 63.6 ±â€¯12.7 ng/L, respectively (p < 0.01). Intrusion of chlorinated soil contaminants resulted in contamination of indoor air above the current accepted indoor air level for PER of 250 µg/m3. For PER in end-exhaled air an accumulation over the workweek was not observed.

Changes in Work Practices for Safe Use of Formaldehyde in a University-Based Anatomy Teaching and Research Facility.

Anatomy teaching and research relies on the use of formaldehyde (FA) as a preservation agent for human and animal tissues. Due to the recent classification of FA as a carcinogen, university hospitals are facing a challenge to (further) reduce exposure to FA. The aim of this study was to reduce exposure to FA in the anatomy teaching and research facility. Workers participated in the development of improved work practices, both technical and organizational solutions. Over a period of 6 years mitigating measures were introduced, including improvement of a down-flow ventilation system, introduction of local exhaust ventilation, collection of drain liquid from displayed specimens in closed containers and leak prevention. Furthermore, some organizational changes were made to reduce the number of FA peak exposures. Stationary and personal air sampling was performed in three different campaigns to assess the effect of these new work practices on inhalation exposure to FA. Samples were collected over 8 h (full shift) and 15 min (task-based) to support mitigation of exposure and improvement of work practices. Air was collected on an adsorbent coated with 2,4-dinitrophenylhydrazine (DNPH) and analyzed by HPLC-UV. Geometric mean (GM) concentrations of FA in the breathing zone over a work-shift were 123 µg/m³ in 2012 and 114 µg/m³ in 2014, exceeding the workplace standard of 150 µg/m³ (8 h time-weighted average, TWA) on 46% of the workdays in 2012 and 38% of the workdays in 2014. This exposure was reduced to an average of 28.8 µg/m³ in 2017 with an estimated probability of exceeding the OEL of 0.6%. Task-based measurements resulted in a mean peak exposures of 291 µg/m³ in 2012 (n = 19) and a mean of 272 µg/m³ in 2014 (n = 21), occasionally exceeding the standard of 500 µg/m³ (15 min TWA), and were reduced to a mean of 88.7 µg/m³ in 2017 (n = 12) with an estimated probability of exceeding the OEL of 1.6%.

Chemical Characterization of the Indoor Air Quality of a University Hospital: Penetration of Outdoor Air Pollutants.

For healthcare centers, local outdoor sources of air pollution represent a potential threat to indoor air quality (IAQ). The aim of this study was to study the impact of local outdoor sources of air pollution on the IAQ of a university hospital. IAQ was characterized at thirteen indoor and two outdoor locations and source samples were collected from a helicopter and an emergency power supply. Volatile organic compounds (VOC), acrolein, formaldehyde, nitrogen dioxide (NO2), respirable particulate matter (PM-4.0 and PM-2.5) and their respective benz(a)pyrene contents were determined over a period of two weeks. Time-weighted average concentrations of NO2 (4.9-17.4 µg/m³) and formaldehyde (2.5-6.4 µg/m³) were similar on all indoor and outdoor locations. The median concentration VOC in indoor air was 119 µg/m³ (range: 33.1-2450 µg/m³) and was fivefold higher in laboratories (316 µg/m³) compared to offices (57.0 µg/m³). PM-4.0 and benzo(a)pyrene concentration were lower in buildings serviced by a >99.95% efficiency particle filter, compared to buildings using a standard 80-90% efficiency filter (p < 0.01). No indications were found that support a significant contribution of known local sources such as fuels or combustion engines to any of the IAQ parameters measured in this study. Chemical IAQ was primarily driven by known indoor sources and activities.

Influence of combined dust reducing carpet and compact air filtration unit on the indoor air quality of a classroom.

Primary schools mostly rely on natural ventilation but also have an interest in affordable technology to improve indoor air quality (IAQ). Laboratory tests show promising results for dust reducing carpets and compact air filtration systems but there is no information available on the performance of these interventions in actual operating classrooms. An exploratory study was performed to evaluate a combination of the two systems in a primary school. Measurements of PM-10 and PM-2.5 were performed by filter sampling and aerosol spectrometry. Other IAQ parameters included black smoke (BS), volatile organic compounds (VOC), nitrogen dioxide (NO2) and formaldehyde. Both interventions were introduced in one classroom during one week, using another classroom as a reference. In a second week the interventions were moved to the other classroom, using the first as a reference (cross-over design). In three remaining weeks the classrooms were compared without interventions. Indoor IAQ parameters were compared to the corresponding outdoor parameters using the indoor/outdoor (I/O) ratio. When the classrooms were occupied (teaching hours) interventions resulted in 27-43% reductions of PM-10, PM-2.5 and BS values. During the weekends the systems reduced these levels by 51-87%. Evaluations using the change in I/O ratios gave comparable results. Levels of VOC, NO2 and formaldehyde were rather low and a contribution of the interventions to the improvement of these gas phase IAQ parameters was inconclusive.

Aggregate dermal exposure to cyclic siloxanes in personal care products: implications for risk assessment.

Consumers who use personal care products (PCPs) are internally exposed to some of the organic components present of which some may be detected in exhaled air when eliminated. The aim of this study was the quantitative determination of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in end-exhaled air to study dermal absorption of substances in PCPs. We exposed the forearm of fifteen healthy volunteers for 60min to pure D4 or D5 and to commercial products containing D4 and D5. Inhalation uptake was kept to a minimum by keeping the forearm in a flow cabinet during dermal exposure and supplying filtered air to the breathing zone of the volunteer during the post-exposure period. End-exhaled air was collected using a breath sampler (Bio-VOC), transferred to carbograph multi-bed adsorbent tubes and analyzed by thermal desorption gas chromatography mass spectrometry (TD-GC-MS). In the end-exhaled air of non-exposed volunteers background concentrations of D4 (0.8-3.5ng/L) and D5 (0.8-4.0ng/L) were observed. After exposing the volunteers, the level of D4 and D5 in end-exhaled air did not or barely exceed background concentrations. At t=90min, a sharp increase of the D4/D5 concentration in end-exhaled air was observed, which we attributed to the inhalation of the substances during a toilet visit without using inhalation protection devices. When this visit was taken out of the protocol, the sharp increase disappeared. Overall, the results of our study indicate that dermal absorption of D4 and D5 contributes only marginally to internal exposure following dermal applications. As in our study inhalation is the primary route of entry for these compounds, we conclude that its risk assessment should focus on this particular exposure route.

Sensitive method for quantification of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in end-exhaled air by thermal desorption gas chromatography mass spectrometry.

Octamethylcyclotetrasiloxane (D4) and decamethylpentasiloxane (D5) are used as ingredients for personal care products (PCPs). Because of the use of these PCPs, consumers are exposed daily to D4 and D5. A sensitive analytical method was developed for analysis of D4 and D5 in end-exhaled air by thermal desorption gas chromatography mass spectrometry (TD-GC-MS), to determine the internal dose for consumer exposure assessment. Fifteen consumers provided end-exhaled air samples that were collected using Bio-VOC breath samplers and subsequently transferred to automatic thermal desorption (ATD) tubes. Prior to use, the ATD tubes were conditioned for a minimum of 4 h at 350 °C. The TD unit and auto sampler were coupled to a GC-MS using electron ionization. Calibration was performed using 0-10 ng/µL solutions of D4/D5 and (13)C-labeled D4/D5 as internal standards. The ions monitored were m/z 281 for D4, 355 for D5, 285 for (13)C-labeled D4, and 360 for (13)C-labeled D5. The addition of internal standard reduced the coefficient of variation from 30.8% to 9.5% for D4 and from 37.8% to 12.5% for D5. The limit of quantification was 2.1 ng/L end-exhaled air for D4 and 1.4 ng/L end-exhaled air for D5. With this method, cyclic siloxanes (D4 and D5) can be quantified in end-exhaled air at concentrations as low as background levels observed in the general population.

Influence of a portable air treatment unit on health-related quality indicators of indoor air in a classroom.

During periods of two weeks in February and June 2010 the performance of portable air treatment units (PATUs) was evaluated in a primary school classroom using indicators of indoor air quality. Air samples were collected in an undisturbed setting on weekend days and in an occupied setting during teaching hours. In the first week PATUs were turned off and in the second week they were turned on. On weekend days PATUs reduced indoor levels of PM-10 by 87% in February and by 70% in June compared to weekend days when PATUs were turned off. On schooldays, indoor PM-10 was increased by 6% in February and reduced by 42% in June. For PM-2.5 reductions on weekend days were 89% in February and 80% in June. On school days PM-2.5 was increased by 15% in February and reduced by 83% in June. Turning on the PATUs reduced total VOC by 80% on weekend days and by 57% on school days (but not in June). No influence on formaldehyde, NO(2), O(3) and molds was observed. PATUs appeared to be less effective in removal of air pollutants when used in an occupied classroom compared to an unoccupied setting. Our study suggests that such devices should be tested in real-life settings to evaluate their influence on indoor air quality.

Estrogenic and androgenic activities in total plasma measured with reporter-gene bioassays: relevant exposure measures for endocrine disruptors in epidemiologic studies?

Measurements of estrogenic and androgenic activities in total plasma with Chemically Activated LUciferase gene eXpression (CALUX®) bioassays could provide biologically relevant measures for exposure to endocrine disruptors in epidemiologic studies. The objective of this study was to explore the effects of a variety of sources of potential endocrine disruptors on estrogenic and androgenic activities in total plasma measured by CALUX®. Plasma samples and interview data on sources of potential endocrine disruptors were collected from 108 men with different exposures profiles. CALUX® measurements (BioDetection Services) involved human U2-OS cell lines controlled by the estrogen receptor alpha and the androgen receptor. Mean differences (beta) in 17ß-estradiol equivalents (EEQs) and dihydrotestosterone equivalents (AEQs) between exposure groups were estimated using general linear models. Mean plasma AEQs and EEQs were 9.1×10(-1)ng/ml and 12.0pg/ml, respectively. Elevated AEQs were found in smokers (beta 1.9 (95%CI 0.1-3.6)×10(-1)ng/ml) and heavy drinkers (1.4 (0.2-3.1)×10(-1)ng/ml), and in men occupationally exposed to disinfectants (1.6 (0.3-3.5)×10(-1)ng/ml) or welding/soldering fumes (1.4 (-0.2-2.9)×10(-1)ng/ml). Occupational exposure to pesticides, disinfectants, and exhaust fumes seemed to be associated with increased plasma EEQs: 1.5 (-0.2-3.2)pg/ml, 2.1 (0.2-3.9)pg/ml, and 2.9 (0.6-5.2)pg/ml, respectively. Moderate to high plasma dioxin levels, measured in a subgroup by the dioxin-responsive CALUX®, were accompanied by a 20% increase in AEQs. This is the first study in which CALUX® was used to assess hormone activities in total plasma. Although the results are not yet readily interpretable, they indicate that these measurements can be valuable for epidemiologic studies on endocrine disruptors and give direction for further research.

Determination of exposure to benzene, toluene and xylenes in Turkish primary school children by analysis of breath and by environmental passive sampling.

Benzene, toluene, m/p-xylene and o-xylene (BTX) are toxic volatile organic compounds and ubiquitous air pollutants. Smoking and consumer products are indoor sources of BTX, whereas traffic and industrial activities are primary sources contributing to outdoor levels of BTX. The aim of this study was to characterize exposure of children to BTX by personal air sampling using diffusive samplers and by analysis of end-exhaled air. For this study, 101 children of 10-11 years of age were recruited from four primary schools in Southern Turkey during the warm season (May 2008). Two schools were situated in a residential area near primary and secondary iron and steel works (Payas) and two schools were located in a non-industrialized city (Iskenderun). The children and their parents were visited at home for an interview and to identify possible sources of BTX in the residence. Median concentrations of benzene determined by diffusive samplers were higher in Payas (4.1 microg/m(3)) than in Iskenderun (2.7 microg/m(3), p<0.001). For toluene, no differences were observed, whereas for xylene isomers air concentrations tended to be lower for children living in Payas. The median end-exhaled air concentrations were 8.2, 29, 3.8, and 5.7 pmol/L for benzene, toluene, m/p-xylene and o-xylene, respectively (Payas), and 6.9, 25, 4.9, and 6.0 pmol/L, respectively (Iskenderun). Concentrations of toluene in end-exhaled air were 50% higher in children living with household members who smoked indoors (p<0.05) and benzene in end-exhaled air was more than 3-fold higher for those children who were exposed to tobacco smoke inside a vehicle (p<0.001). End-exhaled concentrations of benzene were also higher in children living in a residence with an attached garage (p<0.05). These exposure modifying factors were not identified when using the results obtained with diffusive samplers.

The occupational exposure of dermatology nurses to polycyclic aromatic hydrocarbons - evaluating the effectiveness of better skin protection.

OBJECTIVES: We studied the uptake of polycyclic aromatic hydrocarbons (PAH) in nurses who apply ointments containing coal tar to patients and investigated the effectiveness of skin protection methods. METHODS: We determined gas-phase PAH on XAD-2 and particle-associated PAH on filters. We also used pads to determine PAH on the skin. Pyrene and benzo(a)pyrene were analyzed by gas chromatography/mass spectrometry and gas chromatography/tandem mass spectrometry; their respective urinary metabolites 1-hydroxypyrene and 3-hydroxybenzo(a)pyrene were analyzed using high performance liquid chromatography with fluorescence detection. RESULTS: We ruled out the inhalation of airborne pyrene and benzo(a)pyrene as the sources of PAH exposure. However, substantial amounts of pyrene and benzo(a)pyrene were observed on the hands of the nurses (median 33.0 and 16.4 ng/cm (2), respectively). Excretion of urinary 1-hydroxypyrene indicated an increased uptake of pyrene in 8 out of 12 nurses. We asked 35 nurses to perform a treatment with gloves followed by a second treatment without gloves. The use of gloves changed the excretion of 1-hydroxypyrene by -0.58 mumol (range -5.1-1.0 mumol), corresponding to a median reduction of 51.5% (P<0.001). Based on this finding, a new protocol was adopted, involving the permanent use of vinyl gloves and Tyvek sleeves. The effectiveness of this protocol was tested against pre-existing work practices and showed a 97% reduction in skin contamination with pyrene and benzo(a)pyrene, and a lowering in urinary excretion of 1-hydroxypyrene of 57%. CONCLUSION: Protecting the skin more stringently reduced pyrene and benzo(a)pyrene contamination of the hands, and lowered urinary excretion of 1-hydroxypyrene.

Uptake of pyrene in a breast-fed child of a mother treated with coal tar.

A woman was treated for atopic dermatitis with coal tar containing ointments. Coal tar containing ointments contain genotoxic polycyclic aromatic hydrocarbons. Over a period of 50 days the accumulated dose of different coal tar containing ointments treatments corresponded to 993 mg of pyrene and 464 mg of benz[a]pyrene. During this treatment she gave breast milk to her 3-month-old daughter. Analysis of urine samples from the breast-fed child showed elevated levels of urinary excretion of a metabolite of pyrene (1-hydroxypyrene, 1-OHP). These levels were in the same range as urinary excretion levels of this metabolite observed in the mother's urine. As no pyrene was observed in breast milk at a limit of determination of 0.0035 micromol/L, transfer of pyrene from mother to child via breast milk is not likely. Also, a low level of 1-hydroxypyrene observed in the mother's milk did not account for the observed urinary excretion levels in the child. It must therefore be assumed that pyrene was transferred from mother to child via another route, presumably direct skin-to-skin or skin-to-mouth contact. Dermatologists should inform their patients who receive treatment with coal tar containing ointments of the risk of transfer of polycyclic aromatic hydrocarbons by skin-to-skin or skin-to-mouth contact.

Highly increased urinary 1-hydroxypyrene excretion rate in patients with atopic dermatitis treated with topical coal tar.

Coal tar preparations, as used in dermatological practice, contain numerous polycyclic aromatic hydrocarbons of which many are proven animal carcinogens. Increased urinary 1-hydroxypyrene excretion in patients with atopic dermatitis treated with topical coal tar preparations has been demonstrated. Little is known about the relationship between the dermal uptake of polycyclic aromatic hydrocarbons on the one hand and the amount of tar applied to the skin, the total body area affected, the condition of the epidermal barrier and the severity of the dermatitis on the other. We compared urinary 1-hydroxypyrene excretion rate with these variables. The urinary 1-hydroxypyrene excretion rate was highly dependent on the total amount of tar applied to the skin and the total body area affected, and less on the severity of the atopic dermatitis or the condition of the epidermal barrier. Exposure to therapeutic doses of coal tar leads to much higher rates of urinary 1-hydroxypyrene excretion than occupational exposure. Because of the potential carcinogenicity of coal tar, as clearly demonstrated both in animal studies and from occupational exposure, careful consideration should be given to the use of coal tar preparations in dermatological practice. However, the risk of short-term high exposure is unknown. Restriction of the use of coal tar should be based on epidemiological studies and/or appropriate risk models taking into account its relative safety established over many years of clinical use.