Release of Polycyclic Aromatic Hydrocarbons and Heavy Metals from Rubber Crumb in Synthetic Turf Fields: Preliminary Hazard Assessment for Athletes

Copyright: © 2015 Marsili L, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Release of Polycyclic Aromatic Hydrocarbons and Heavy Metals from Rubber Crumb in Synthetic Turf Fields: Preliminary Hazard Assessment for Athletes


Introduction
World population increase is accompanied by increasing consumption of resources. This makes recycling of materials extremely important to reduce waste. However, recycling itself is not enough, because it is necessary to understand if recycled materials have adverse effects on humans and environment, such as the case of used tyres, loaded of potentially toxic substances and recycled in synthetic turf. Today, synthetic turf is common in many sporting facilities. Created in the 1950s by the humanitarian Ford Foundation of New York and Chemstrand Corporation, it gained huge success in 1966 when used in the Astrodome stadium, Houston, Texas [1]. In the '70s and '80s, it was applied in many sports grounds in America and Canada, and was introduced into Europe in the mid-1980s. Softer new types of synthetic turf containing polyethylene were developed and introduced all over the world in the late 1990s [2]. Synthetic turfs differ in relation to their method of production and infill technique. Normally, the layer of infill consists of rubber crumb, which in a typical application reaches a thickness of 3 cm, and is spread on a thin layer of sand [3]. The most common source of rubber crumb is recycled tyres (recycled styrene-butadiene rubber -SBRr); the diameter of the crumb can vary between 0.5 and 3 mm [4]. Hazardous substances in crumb rubber infill are primarily, volatile components (nitrosamines, xylenes), benzothiazoles, secondary amines, heavy metals (especially zinc) and polycyclic aromatic hydrocarbons (PAHs) [5]. In particular, the presence of zinc (Zn) is due to zinc oxide that is used as a vulcanization aid in the rubber production process and PAHs come from higharomatic oil that is used as an additive in the production of tyres. In 2005, the Italian Ministry for the Environment allowed SBR crumb for synthetic grass courts in Italy [6], but there are still no European Union guidelines defining measures to protect the environment and human health in relation to SBRr in synthetic turf. The only standard to which manufacturers refer in producing SBRr crumb was published in 2002 by the German Institute for Standardization (DIN) establishing limits for certain heavy metals in soil, but with no reference to PAHs [7]. This standard was also chosen by the Italian National Amateur League (LND) in its "Regulations for the construction of latest generation artificial turf football fields", which defines soccer field parameters necessary for approval and use. Besides purely technical qualities, it also includes concentration limits for certain substances, including heavy metals and some high molecular weight PAHs (limits reflect those provided by Legislative Decree 152/2006 [8,9]. Sport grounds fitted with synthetic turf filled with crumb of recycled tyres may release dangerous particles in air, contaminate soil and groundwater with soluble contaminants leached by rain, and pose health hazards for residents and users due to inhalation of volatile substances [10]. Some coats for rubber granulates can effectively reduce emissions in the environment of these contaminants but they are not systematically used [11]. Though designed for sporting facilities, it is not uncommon to find synthetic grass in recreational parks and children's playgrounds. Synthetic turf may reach high temperatures: for example, on a day with an air temperature of 26°C in the early afternoon, synthetic surfaces may reach 60°C, making it difficult to play on them [2]. The U.S. Center for Disease Control and Prevention (CDC) has not yet assessed the risks associated with exposure to dust released by rubber crumb from playing fields. As a precautionary measure, it issued general recommendations for users to minimize any potential risk, such as wash aggressively hand and body after playing, do not eat and drink on the field and do not use clothes and shoes after the activity for normal life [12]. Some studies have focused on levels of heavy metals, such as Zn, or PAHs in tyre rubber, both in granulates and in leachate [3,[13][14][15] evaluating also the ecotoxicological effects in different organisms and humans [14,[16][17][18][19].
In response to concern about human exposure through direct contact or inhalation, the principal aims of this study were: 1) to quantify the PAHs and heavy metals contained in rubber crumb from recycled tyres, produced before 2010 [20]. used in synthetic turf, to determine whether PAHs are released and at what concentrations, becoming bioavailable to synthetic turf users at high temperatures; 2) to estimate respiratory uptake by athletes training on these grounds.

Sample collection
Samples of nine different synthetic turfs from football fields in Tuscany and Lazio (Italy) were analyzed. Samples 1 to 5 were new and had not yet been spread on playing fields yet; samples 6 to 9 were obtained from fields that had been laid down for 1 to 8 years. The crumb of sample 5 was virgin rubber and not recycled tyres (Table 1). In the laboratory, the samples were kept at room temperature, in black bags, away from sunlight.

Heavy metals analysis
The samples were mineralized in a microwave oven (EPA Method 3052 modified in the Lab. (Bianchi, p.c.)). About 0.3 g of rubber crumb sample was placed in Teflon containers, spiked with 8 mL nitric acid (HNO 3 ) and 2 mL hydrogen peroxide (H 2 O 2 ), then transferred to a microwave oven. The solutions thus obtained were cooled to a final volume of 50 mL and concentrations of lead (Pb), copper (Cu), nickel (Ni), zinc (Zn), chromium (Cr), cadmium (Cd) and iron (Fe) were evaluated. A blank was included in each series to check the purity of reagents and two tests of reference materials (ERM-EC680k and NIST-SRM2710) with concentrations certified by the Community Bureau of Reference were performed to check analytical accuracy. Cr, Cu, Ni, Pb and Cd concentrations were determined with PerkinElmer AAnalyst700 high-performance atomic absorption spectrometers with graphite furnace. Zn and Fe concentrations were determined with an Analytik Jena ContrAA700 acetylene flame atomic absorption spectrophotometer. All metal concentrations were expressed as the mean of three replicates in μg/g on a dry weight basis.

PAH analysis
PAH extraction in rubber crumb: PAHs were extracted according to Griest and Caton [21,22] and Holoubek et al [22]. with some modifications [23]. About 1.0 g of rubber crumb was extracted with a mixture of KOH 2M/methanol (1:4) in a Soxhlet apparatus for 4 h at 75°C. The mixture was extracted by shaking in separator funnels with 200 mL of cyclohexane. Liquid/liquid separation was performed to bring the PAH fraction into the supernatant. The liquid recovered was concentrated in a Rotavapor system, resuspended with 10 mL acetone/ hexane (1:1) and purified in a chromatographic column packed with 3 cm of Florisil, about 60-100 US mesh, previously set at 120°C for 2 h. Elution was carried out with 90 mL acetone/hexane (1:1). The organic fraction was concentrated and suspended in 0.5 mL acetonitrile for HPLC analysis.

PAH Extraction in evaporates of rubber crumb:
Since synthetic fields can reach 60°C when the air temperature is about 25°C, a method to evaluate release of PAHs at this temperature was used. Small flasks (25 mL) were filled with a quantity of rubber crumb up to 3 cm high, in order to simulate their thickness in a synthetic field. Then, the following steps were applied: 1) a closed trap packed with a bottom layer of cotton/ fiberglass and a 3 cm layer of Florisil, previously activated at 120°C for 2 h, was placed on every flask; 2) the flask/trap system was kept at 60°C for about 5 h (assumed to be the average period at 25°C in a day) to capture the evaporates of rubber crumb; 3) liquid chromatography was then immediately performed using the trap as a column by pouring in 10 mL acetone/hexane (1:1) and then a further 90 mL of the same mixture; 4) the extract thus obtained was concentrated in a Rotavapor system and resuspended in 0.5 mL acetonitrile for HPLC analysis. This procedure (steps 1-4) was repeated three times to obtain three consecutive readings for each sample, thus determining whether or not the release of PAHs was continuous. The efficiency of the traps was validated with two different evaporation tests: first evaporation of the standard EPA 610 in acetonitrile (1/100) and, second evaporation, with the same amount of EPA 610 (1/100) mixed with a rubber crumb sample 3 cm high. HPLC analysis showed that the efficiency of the traps was about the 90%. In fact, in the first case, summing the amount of PAHs found in the evaporated to those found in the sample left in the flask, the value was almost like to the original amount of the standard EPA 610. In the other case of the standard mixed with the rubber crumb, there was a little matrix effect because the amount of PAHs found in the evaporated was lower (5-10%) than those found in the evaporated of the standard alone.
PAH analysis: PAHs were analyzed by an HPLC/fluorescence system. PAHs were separated using a reversed-phase column (Supelcosil LC-18, 25 cm × 4.6 mm i.d., 0.5 μm particle size, pore size 120Å) with an acetonitrile/water gradient from 60% to 100% acetonitrile for 20 min, then isocratically for 10 min. The flow rate was 1.5 mL/min. The mobile-phase was degassed with a helium stream. An external standard consisting of 16 PAHs from Supelco (EPA 610) was used. Fourteen PAHs were analyzed and the results expressed in ng/g. Recoveries were 80-98%. The detection limit, calculated at a signal-tonoise ratio of three, was 0.1 ng/g for all PAHs. Assay reproducibility was determined by five replicate analyses of a single sample: the coefficient of variation was 1-3%, depending on the compound. Blanks contained undetectable amounts of PAHs.

Results and Discussion
Heavy metal concentrations in rubber crumb Table 2 shows the concentrations of heavy metals (cadmium, lead, chromium, nickel, copper, zinc and iron; mg/kg) in rubber crumb samples and the maximum admissible concentration set by the Italian National Amateur League [9]. These limits are identical to those of Dlgs. 152/2006 [8] for public parks and private and residential land.
Lead, chromium, nickel and copper were well below the limits in all samples. Three samples exceeded the limit for cadmium, two being new (samples 4 and 5) and the third already installed (sample 6). In the case of zinc, all samples recorded high concentrations: sample 1 showed the lowest concentration of zinc, exceeding the limit by a factor of more than 20; the worst case was sample 4, exceeding the limit by a factor of nearly 90. Concentrations were quite similar to those of the study of Bocca et al. [13], except for cadmium that was always below the limit in the cited study. Concentrations of iron were quite similar to each other, except for sample 2 that showed a particularly high peak.
Zinc values are in line with other studies concerned with it: Verschoor [3] not only assessed the quantity of zinc in the rubber infill, but also the amount released, showing that the aging of rubber has a high impact on the release of zinc, which the author estimated as an annual average of 50 mg/kg of rubber. The concentration of Zn found in leachate was 1.3 mg/L, which is above the limit imposed by the Dutch Soil Quality law [24,25].
Another study evaluated the bioavailability of certain metals and PAHs in human digestive fluids, assuming ingestion of crumb from synthetic fields [26]. Their results showed that Zn in particular exceeded the limits of the Department of Environmental Conservation (DEC) of New York State [27] for soil (2200 ppm), while the lead content in rubber crumb never exceeded these limits, but was very bioavailable in synthetic gastric fluid, thus representing a potential risk to athletes.

PAH levels in rubber crumb
It proved possible to identify and quantify the PAHs in all samples. All were priority PAHs according to USEPA [28] and some are known to be powerful carcinogens (Table 3) [29][30][31][32]. The total PAHs in tables and graphs are the sum of individual PAHs, while the carcinogenic PAHs are only those which are carcinogenic according to at least three classifications. Although benzo(a)pyrene (B[a]P) only accounted for 10-20% of the carcinogenic compounds, it is used by the European Commission Regulation 1881/2006 as an indicator of contamination by the 16 priority PAHs [33]. Table 4 shows the levels of single PAHs (ng/g) detected in samples 1 to 9. Samples 1 to 5 were obtained before they were spread on playing surfaces, whereas the samples 6 to 9 were collected directly from the fields and had been in place for 1 to 8 years. The two last rows of Table  4 show total PAH levels, obtained by summing all the PAHs quantified, and carcinogenic PAH levels, obtained by summing carcinogenic PAHs of Table 3. Figure 1 compares levels of total PAHs and carcinogenic PAHs in rubber crumb from the various football fields. Very high levels of total PAHs were found in samples 2, 9 and 1. Lower levels were found in samples 3, 6, 7 and 8, indicating a difference between new samples and those already installed in soccer fields. Indeed, the load of PAHs was appreciably lower in samples 6, 7 and 8 (installed 3 to 8 years ago) than in the other samples. This shows that once installed, these fields lose part of their toxic load in the time. This fact is important for assessing toxicological hazard to athletes, therefore they are chronically exposed to these compounds.
Comparing the relative percentages of all PAHs on total PAHs of the different samples ( (Table 4), the three most abundant PAHs (Fl, Pyr and Flt) are not regarded as particularly hazardous or carcinogenic to humans and therefore this type of natural rubber crumb can be considered less toxic. Table 5 shows the levels of those PAHs (mg/kg) of which the maximum admissible concentration is established [8], detected in the rubber crumb samples analyzed for this study. The Decree reported threshold values of concentration for some PAHs in soils and even if the comparison with the present data was not direct it could give some indications.  Table 6 shows the levels of benzo(a)anthracene (B[a]A), Chry, B[a] P and B[ghi]Per, among the most toxic high molecular weight PAHs detected in evaporates of nine rubber crumbs. We have taken only these PAHs because, when the evaporation test was repeated three times to obtain three consecutive readings for each sample, were the only PAHs which standard deviation (S.D.) was below the mean value in all nine samples. Among all samples, turf fields 9 and 1 released particularly high levels of all considered compounds. Evaporation tests showed that the releasing of four PAHs into the air by rubber crumb did not decrease with the time, suggesting chronic contamination in areas fitted with synthetic turf filled with rubber crumb.

PAH levels in evaporates of rubber crumb
It was also evaluated the mean times for total release of these four PAHs from the samples ( Table 7). In theory, considering for each compound the total amount present in the rubber crumb samples and the amount found in the evaporated samples, we can estimate that for the new turf fields are necessary from a minimum of 811 times (sample 3) to a maximum of 4423 times (sample 2) to exhaust emissions of these compounds when the turf temperature reaches 60°C, then when the atmospheric temperature is 25°C. Regarding the used samples, in the same conditions of temperature, are required from a minimum of 346 times (sample 7) to a maximum of 655 times (sample 6). Assuming solar radiation keeps atmospheric temperature at 25°C for at least 5 h/day (heating experimental time in the Lab.) for 5 months of the year, there are 150 suitable days per year. Ignoring other sources of elimination, such as rainwater or washing that cause leaching and cooler days when the crumb still becomes warm, it would hypothetically take a minimum

Risk assessment for PAH inhalation from synthetic fields
The approach to assess human health risks through the inhalation route in the synthetic turf fields, plans to consider field surface as soil surface [24]. Then, if field surface does not reach a temperature of 25°C, the contaminant release in air can be associated to wind erosion and volatilization and the inhalation risk must consider also the contaminated dust resuspension. This site-specific inhalation risk evaluation that we have conducted on the rubber granulates of the nine synthetic fields, follows the recommendations of the Dlgs. 152/06 [8] and the indication of the technical procedure issued by APAT [34]. This evaluation procedure is applied separately on each pollutant and, at the end, all individual risk values obtained are summed. In order to proceed to this estimation, it is important to know the particulate emission factor (PEF) of outdoor matter of the survey site. We considered PEF equal to PM10×10 -6 kg/mg, where PM10 are the levels of total inhalable dusts (mg/m 3 ) potentially containing PAHs, assuming that all the particles present in the air as PM10 result from the volatilization of particles from the field, and not as an input from the wide variety of anthropogenic and crustal sources. Then, the following results are overestimated and they must be considered as extreme worst case screening. The precautionary principle is applied taking into account the highest average concentration of PM10 recorded in 2010 in Tuscany (since 8 turfs of 9 came from Tuscany) which is PM10 = 0.0517 mg/m 3 . Considering a punctiform source of contamination, it is possible to evaluate the contaminant concentration in air (C A ) given by C A = contaminant concentration in field (C F ) (mg/kg) × PEF. In this way it is possible to calculate the Average Daily Dose (ADD), assumed by the athletes, expressed in terms of mass of contaminant per unit of body weight per day (mg/kg day). The ADD is calculated to evaluate toxic effects taking into consideration the C A values, the inhalation rate (IR) of an athlete Starting from ADD and LADD values, in the last step it is possible to calculate a Hazard Quotient (HQ) as an indicator of risks associated with health effects other than cancer, and Excess Cancer Risk (ECR) as the incremental probability of an exposed person developing cancer over  (Table 8A-B). HQs for all PAHs are summed to provide an overall Hazard Index (HI). When HI ≤ 1 there are no concern for potential adverse systemic health effects in the exposed individuals. Summing the individual ECR for all PAHs, it provides the Cumulative Excess Cancer Risk (∑ECR), that is acceptable if < 10 -6 [36]. In the different rubber granulates samples was found a HI range that varies between a minimum of 8.94×10 -7 in sample 4 and a maximum of 1.16×10 -6 in sample 1 (Table 8A). The ∑ECR range goes from a minimum of 4.91×10 -9 for sample 6 to a maximum of 1.10×10 -8 for sample 1 again (Table 8B). All values were considered as acceptable. Menichini et al. [37] found an excess lifetime cancer risk of 1×10 -6 for an athlete with an intense 30-years activity; then despite the different parameters considered for the athlete in this study, the results are very similar.
In reality, when the ambient temperature is 25°C and direct sunlight exposure is present on field, rubber granulates reach a mean temperature of 60°C, where a chronic release of PAHs occur, as seen in section 3.3. According to the high evaporation which occurs in this condition and knowing that these fields are used anyway with such temperatures, despite it should be decreased by watering, we calculate an estimate of risk for outdoor fields at 60°C. Applying again the precautionary principle estimating the maximum risk, we consider only the central area of the field of size 18 × 32 m (576 m 2 ), from now indicated as Interested Area (IA) (Figure 3), where the exchange of air at 2 m is irrelevant because it comes from surrounding perimeter and then it has the same toxicological characteristics. The air temperature at 2 m above the field is considered to have the same temperature as the granulates, according to the principle of the vertical temperature gradient. First, we calculated the quantity of crumb in a soccer field, averaging the specific weights (γ), which were similar, of the various samples analyzed (mean 0.518 g/mL). Considering IA paved with synthetic turf 3 cm thick, the quantity of crumb of specific weight 0.518 g/mL is 8951 kg. To estimate risk to human health from exposure to PAHs, we expressed the toxicity of the various PAHs with respect to B[a]P, in other words as Benzo(a)Pyrene Equivalent (BaP eq ). We calculated the Toxic Equivalent Quantity (TEQ) by multiplying the individual PAH levels in evaporates by their Toxic Equivalency Factor     IA use 8951 kg of rubber crumb and we estimated the TEQ in μg referred to the crumb evaporates of the different fields at an average air temperature of 25°C (Table 10A). Estimating evaporation up to a height of 2 m, we have a volume of 1152 m 3 (576 m 2 × 2m). Table 10B shows the results in µg/m 3 obtained dividing the TEQ of evaporates of the rubber crumb samples (µg) by the estimated volume of air (m 3 ) above the field. Knowing that an athlete inhalation rate is around 3.6 m 3 per hour [41], the TEQ inspired by him in a standard two-hour workout was calculated on the basis of that assumption. In 2 h of training, the daily intake of BaP eq of an athlete is showed in Table 10C. If a player trains for 2 h a day, three times a week, five times for professionals, plus the match, his estimated intake of PAHs as TEQ ranged from 31.2 μg/ week (sample 4) to 219.2 μg/week (sample 9), for an average weekly exposure of 8 h (Table 10D). Dividing this by 7 days we obtain 4.46 -31.3 μg/day of BaP eq inhaled as daily mean dose, not considering other PAH inputs for the athletes (Table 10E). For a 70 kg athlete, we obtain an intake of 0.06 -0.45 μg/kg bw of BaP eq per day (Table 10F). Since the release of PAHs is continuous and constant throughout the life of the field (Table 6), a chronic exposure of 0.06 to 0.44 μg/kg bw per day BaP eq for a 70 kg athlete should not be underestimated. In fact recent studies have shown that 0.57 -5.00 ng/kg bw per day is a virtually safe dose of B[a]P in food, which implies a risk of 1x10 -6 (one person in a million will develop cancer after chronic exposure). Considering that, generally, carcinogenic PAHs are about 10-fold higher than the B[a] P alone, the carcinogenicity increases and a virtually safe dose of B[a] P, as an indicator of carcinogenic PAHs in food, would be in the range 0.06 -0.50 ng/kg bw per day [42], theoretically 1000 times lower than the range of 0.06 to 0.45 μg/kg bw per day found in this study.

Conclusions
Rubber crumb derived from recycled tyres, like the tyres themselves, should be considered non-hazardous special waste. The literature and the present study show that crumb contains PAHs and heavy metals. Fine dust may become airborne and leachate may filter into the soil. The magnitude of human exposure depends on chemicals of concern concentration in field, exposure parameters describing human physiology (e.g. dermal contact, body weight) and population-specific parameters describing exposure behaviour (exposure frequency, duration). Randomly ingested crumb may release these compounds in the digestive tract. Most of all, evaporation at high temperatures may expose users of sports grounds, who are often children between 5 and 13 years of age, in a very sensitive phase of growth, to many of these toxic compounds.
The results of the present study demonstrate that PAHs are continuously released from rubber crumb through evaporation. Athletes frequenting grounds with synthetic turf are therefore exposed to chronic toxicity from PAHs. The main conclusion we can draw from this preliminary study, which will be validated by further field and laboratory research, is that although synthetic turf offers various advantages over natural grass, the quantity of toxic substances it releases when heated does not make it safe for public health. When we extrapolated the data obtained in laboratory, the toxicity equivalent (TEQ) of the different compounds evaporating from the crumb was far from negligible and would contribute substantially to an athlete's total daily PAH intake. In fact, all rubber crumb samples of this study exceeded the Dlgs. 152/2006 [8] for B[b]F, B[ghi]Per and Zn, but all PAHs, except Chry, were over the threshold in almost one synthetic field. It must be underlined that this preliminary hazard assessment overestimates the PAH contribution of the field because the input from the wide variety of anthropogenic and crustal sources were not considered and then, this theoretical approach must be considered as an extreme worst case screening.  Table 10. Toxicity Equivalent (TEQ) in benzo(a)pyrene equivalents (BaP eq in µg): A) theoretically released by rubber crumb in the Interested Area (IA); B) per m 3 of air over IA; C) inhaled by an athlete in 2 h of activity; D) inhaled by an athlete in an average weekly exposure (8h) of activity; E) inhaled by an athlete as daily mean dose and F) a daily intake for a 70 kg athlete.