Urinary metabolites of VOCs were detected in all infants in this study; despite the small sample size of this study, these results suggest widespread exposure of infants in the NICU to some VOCs. This is not surprising as VOCs are ubiquitous in indoor environments. Neonates in incubators showed increased levels of urinary metabolites consistent with higher levels of exposure to parent compounds such as acrolein, acrylonitrile, carbon disulfide, cyanide, N,N-dimethylformamide/methyl isocyanate, ethylbenzene, ethylene oxide, propylene oxide, styrene, toluene/benzyl alcohol, vinyl chloride, and xylene compared with infants in open cribs. These higher levels may be due to the lower gestational age/corrected age and slower metabolism in the less-mature infants in the incubators (2,13). In contrast, two urinary metabolites were elevated in the open-crib group, consistent with higher 1,3-butadiene exposure. Potential sources of these 12 VOCs in a NICU include tobacco smoke (from parents’ and caregivers’ clothes and hair) (14), plasticizers from medical devices (5), excipients used in medications (15), and the cleaning/ sterilizing agents used to clean the cribs and incubators in particular and the NICU in general (16,17).

Although certain VOC metabolites (CEMA, 3HPMA, AAMA, GAMA, CYMA, HEMA, DHBMA, MHBMA3, HPMMA, ATCA, AMCC, PGA, MA, and 3MHA+4MHA) identified in the incubator and/or open-crib groups share cigarette smoke as a common source (10), the precise source of potential exposure within the NICU was not investigated in the present study. A history of maternal cigarette smoking is routinely reported in the medical records in only 10% of the mothers of infants studied. The number of mothers who reported smoking was not found to be statistically significant between the crib and incubator groups. However, assessment of smoking using medical record data may be inaccurate, and our sample size may not be large enough to detect a difference in smoking between the crib and incubator groups. Also, the median day of life of urine sample collection was 11 and 16 days for infants in the incubators and cribs, respectively. This makes the detection of VOC metabolites related to prenatal maternal cigarette smoking unlikely. Although maternal smoking was not more common in the incubator group, it is possible that any cigarette smoke on the garments of a NICU caregiver or visiting parent may have off-gassed and becomes trapped within the enclosed incubator to cause increased concentrations of VOCs. On the other hand, infants in open cribs may have more physical contact with NICU staff and parents, potentially increasing exposure to VOC sources such as cigarette smoke on the clothing of handlers.

Plasticizers, acrylics, and synthetic rubber are common materials in the NICU. VOCs may be emitted from polyurethane foam in bedding materials (1,5,6,18).

Another possible source of VOCs inside the incubators is the cleaning and disinfecting agents, including ethylene oxide, which is commonly used as a disinfectant for respiratory equipment (16,17). There is also the possibility that VOCs inside the incubators originate from external sources, such as cleaning and medical care products that become part of the internal breathable air inside the incubators (19,20). However, as only a subset of VOCs was found to be higher in incubator infants compared with crib infants, other sources unique to individual VOCs must be considered.

Potential exposure to toluene deserves special mention. Toluene is a known teratogen and is associated with toluene embryopathy in infants whose mothers were highly exposed to solvents (occupationally or from solvent abuse). Although toluene exposure in these infants was orders of magnitude lower than that found in solvent abusers (21), the geometric means for the toluene metabolite BMA in both the crib and incubator groups were 7 and 12 times higher than that of NHANES for 6–11-year olds. Indeed, the geometric means were 1.9 and 3.1 times the NHANES that reported the 95th percentile for 6–11-year olds (95% CI 33.1 ng/ml (23.7–56.4 ng/ml)) (10). The highest concentrations measured in the crib group (479 ng/ml) and in the incubator group (388 ng/ml) indicate exposures of an order of magnitude higher than the 95th percentile of 6–11-year olds. However, BMA is a relatively nonselective biomarker; for example, benzyl alcohol can be metabolized to BMA (22). Benzyl alcohol, a known toxicant to preterm infants, is still used as an excipient in the NICU (15). The higher concentrations of BMA in the infants compared with NHANES children may be due to the benzyl alcohol used as an excipient in the medications that they receive while in the NICU. Additional work is needed to characterize toluene exposure for NICU infants, to disentangle the sources of the metabolite (i.e., toluene or benzyl alcohol), and to better understand whether the levels measured in this sample population are representative of infants at other locations.

The results of this study are presented with several caveats. First, parent VOCs may be broken down to form more than one metabolite and some metabolites may be derived from more than one parent VOC; not all biomarkers measured in this study are unique to an individual parent VOC, as shown in Table 2. Thus, an elevated level of an individual metabolite may not represent exposure to a single compound. Second, we did not control for individual hydration since all babies in our NICU receive the same amount of fluid per day whether by parenteral or enteral nutrition. Further, interpretation of the sources of VOCs is difficult as unmeasured variables may have contributed to differences in VOC exposure, such as time between manufacturing and use of materials, and the smoking habits of hospital caregivers and visitors such as fathers and mothers. Another source of uncertainty that could impact urinary VOC levels is the time interval between sanitization of the incubators and cribs and placement of an infant into the incubator, with an off-gassing period ranging from 45 min to days. A 2-week period to allow for dissipation of these gases has been suggested (23).

In addition, we did not collect information needed to evaluate whether the differences in VOC levels between the two infant groups were related to the differences in metabolism rates between the groups. The incubator and open-crib groups have several demographic differences, including body weight and gestational age at sampling (Table 1), and these factors are related to the decision to place the infant in an incubator or open crib during their care in the NICU. The significantly lower gestational and post-conceptual age of the incubator group could contribute to lower excretion of the VOC metabolites in this group due to functional immaturity of the renal glomeruli and the excretion system of the smaller infants (13). However, other important differences are also present that we did not measure, including the presence or absence of bronchopulmonary dysplasia (a disease known to influence VOC production) (24), amount and type of medications (15), and dietary differences (breast milk vs. formula), all of which could influence the urinary excretion of VOCs. Therefore, we cannot directly extrapolate the quantity of parent compound to which an infant was exposed from the quantity of urinary VOC metabolites measured from these infants. The proportion of parent compound VOC metabolized and excreted into the urine vs. the proportion distributed into the infant’s tissues and the remaining bioreactive VOCs are unknown for the compounds analyzed here and may vary depending on an individual infant’s metabolic characteristics (25). Finally, it is possible that some of the metabolites measured here could also be derived as a metabolic intermediate from an unknown VOC or an endogenous process.

In this study, we demonstrate the feasibility of measuring infant exposures to VOCs in a NICU setting. We further demonstrate that infants have exposures to a wide array of VOCs in both cribs and incubators and that exposures may vary depending on which type of unit an infant is placed. The findings indicate the need for a study with a larger sample size to increase the power of the study, multiple samples per infant to better characterize the variability in exposure, and more data on products used in the NICU to better understand the sources of VOC exposure. Such a follow-on study would help to determine the validity of these findings and to identify possible hospital practices that could be modified to reduce validated exposures. One such practice would be to increase the ventilation rate of the incubator. Current specifications for one incubator are an airflow of 0.2–2.0 m/s. Calibration could be increased to the higher end of this range. Further studies are needed to understand the clinical significance of VOC exposure within the NICU and the impact on long-term health, including both respiratory and neurodevelopmental outcomes.