The similarity of variance ratios from two independent populations lends credibility to the findings reported in Chapter 7 and suggests that the levels of variability observed in concentrations of chemicals measured in dust from the Fresno Exposure Study residences may be generalized to other populations. In using the random-effects model to estimate variance components, it is implicitly assumed that each residence has a true underlying dust concentration for each chemical that remains constant over the course of the study [i.e., exp, equivalent to the geometric mean concentration in the i th household]. As such, any deviation from a residence’s true level is interpreted as measurement error or random within-residence variability. It is possible that some of the “random” variability that was observed is due to changes in the sources of chemical contamination in homes during the course of the study. Indeed, since the Fresno Exposure Study dust samples were collected over the period of 3 years, it is possible that true concentrations of chemicals in household dust changed systematically over time. Consequently, the long-term timing of the dust sampling could have artificially inflated the within-household variance component, resulting in an overestimate of the variance ratios and the associated attenuation bias. Nevertheless, the random-effects model should provide a conservative estimate of the reliability of chemicals in residential dust as measures of exposure. Indeed,vertical farming systems the results from Chapter 7 indicate that residential dust would be a valuable tool for retrospective exposure assessment given the modest within-residence variability observed for dust measurements collected several months apart.
A limitation of the method used for predicting attenuation bias is the implicit assumption that measurement error is non-differential . In the more complex situation where case and control populations have different variance ratios, Equation 3 would be only approximate. In summary, estimates of variance ratios for concentrations of PAHs , PCBs , and nicotine measured in residential dust were modest for the 21 homes in the Fresno Exposure Study. Though based on a limited number of measurements , these findings suggest that the use of residential-dust measurements as markers of exposure to these 13 chemicals will result in relatively small levels of attenuation bias due to exposure measurement error. Likewise, these results indicate that residential dust would be a valuable tool for retrospective exposure assessment. In multi-variable regression analyses, it was observed that the age of the residence was a consistent determinant of the concentrations of nicotine , PAHs , and PCBs in residential dust. This conclusion was supported by evidence that dust from homes of former smokers had higher nicotine concentrations than dust from homes of nonsmokers , indicating that residual nicotine contamination could persist in dust for years after smoking cessation. Likewise dust levels of PCBs were higher in homes constructed before 1980 than in newer homes , demonstrating that these chemicals remained inside homes more than three decades after PCBs were banned in the U.S. The persistence of chemicals in residential dust offers an advantage for the estimation of long-term chemical exposures compared to air or biological measurements, which generally provide only a “snapshot” of current chemical exposures . As such, chemicals measured in residential dust may be useful measures of cumulative exposure in epidemiologic studies, especially for diseases with long latency periods. Another advantage of using residential-dust levels as measures of chemical exposure is the fact that dust levels provide information that questionnaires cannot. Whereas dust measurements are specific and quantitative indicators of chemical contamination in the home, questionnaires generally offer only qualitative measures of chemical exposure.
Indeed, even the extensive survey information developed by the NCCLS offered only weak predictions of residential-dust levels of PAHs or PCBs and modest predictions of dust levels of nicotine . Since questionnaire-based exposure surrogates appear to be poor predictors of indoor levels of chemical contamination, they are also likely to be poor predictors of chemical exposure. Thus, it is recommended that residential dust measurements be used instead of, or in addition to, questionnaires when evaluating exposures to chemicals. Another benefit of using residential dust for exposure assessment is the simple, inexpensive, and non-invasive nature of sample collection. By using dust from household vacuum cleaners and collecting samples via the mail, it is possible to obtain dust samples without ever visiting subjects’ homes . In the NCCLS, analyses of chemical levels in dust obtained from household vacuum cleaners were indistinguishable from those obtained with the more rigorous and expensive high volume surface sampler . Finally, residential dust can be an important source of chemical exposures. For example, dust ingestion may be the dominant route of exposure to PBDEs for individuals from North America and Asia , and investigators have observed strong correlations between levels of PBDEs in matched samples of dust and biological specimens have been observed . Although dust exposure appears to play a more minor role in the intake of other chemicals, investigators have still observed correlations between dust concentrations and biomarkers of PCBs and nicotine . Thus, there is evidence that dust measurements can be useful not only as markers of residential chemical contamination, but as surrogates of chemical uptake and dose. Moreover, since dust is a particularly important source of chemical exposure for young children, dust measurements could be especially useful in studies of childhood diseases. It is important to keep in mind that residential dust provides only one piece of the complete ‘puzzle’ of chemical exposure. For example, a dust measurement would not necessarily provide information about a subject’s potential chemical exposures via the inhalation of contaminated air, particularly for volatile substances, or the ingestion of contaminated food. Thus, measuring the concentration of chemicals in residential dust allows an investigator to estimate directly only one route of exposure . In some populations and for some chemicals, contaminated dust may not be a relevant source of chemical exposure, and in these scenarios, dust may not be the ideal medium for assessing exposures.
In some cases, it is appropriate to use chemicals measured in dust as surrogates for total chemical intake, even if dust ingestion is only a minor source of exposure. For example, in Chapter 3 it was shown that residential-dust nicotine measurements could be used as surrogates for secondhand smoke exposures. However, nicotine can be conveyed into the home not only by residential smoking, but also via the skin, clothing, or shoes of smokers . Moreover, nicotine concentrations can remain elevated in dust collected from non-smoking families living in apartments formerly occupied by smokers . As such, dust contamination is not necessarily indicative of exposures received via inhalation or other routes. Residential dust contamination will be an ineffective measure of chemical exposures for some individuals. For example, concentrations of chemicals measured in residential dust from a subject’s current home may not be representative of chemical concentrations in dust from their previous home. As such, it may be difficult to use residential-dust samples to estimate past exposures in residentially mobile study populations. Likewise, residential dust does not provide information about potential chemical exposures that may occur outside of the home, such as exposures received at work, while commuting, or in public spaces. Of course, dust could potentially be collected from these other micro-environments to provide more complete information about chemical exposures, but that would be difficult in most cases. Alternatively,drying marijuana investigators could use residential dust to assess exposures in less mobile subjects , who are mostly exposed to chemicals in their own homes. Some diseases are the result of a chemical exposure during a specific time window of development . However, since residential dust measurements are long-term measures of exposure, it can be difficult to use dust to estimate exposures at a specific time of interest. Investigators that plan to use dust measurements to estimate exposures need to consider the complexity of the analytical method. As described in Chapter 2, the dust measurement protocol requires several preparatory steps prior to GC-MS analysis. In all, the current method requires extensive time, expertise, and instrumentation. Other researchers have used simpler methods for dust analyses, but the method described in Chapter 2 was optimized to minimize GCMS interferences and maximize analyte sensitivity. Since there is limited information regarding how concentrations of chemicals in dust vary across time and space within a residence, it would be useful to use a larger dataset to verify the findings from Chapter 7. Specifically, investigators should determine whether a single dust measurement can effectively represent indoor contamination from the distant past , as would be necessary when performing retrospective exposure assessment. Investigators should also use data from repeated measurements collected from case-control study populations to investigate differences in measurement errors between case and control populations. Since investigators have struggled to find determinants of PBDEs, future researchers should identify factors that impact PBDE levels in residential dust.
Specifically, researchers should investigate the hypothesis that differences in PBDE contamination across California may be a function of inter-community income disparities. It would also be worthwhile to evaluate whether PBDE levels and congener patterns have changed significantly over time as a result of recent restrictions of the commercial Penta-BDE and Octa-BDE mixtures. Finally, future work should assess the degradation of BDE-209 in the environment, as this phenomenon may increase the prevalence of more harmful lower-brominated BDE molecules in dust. Researchers should also follow-up on the findings of Matt et al. , who reported that residual nicotine contamination could persist in apartments of former smokers. Specifically, investigators should use longitudinal data to evaluate changes in nicotine dust levels over time for households with changing smoking habits. For example, it would be useful to assess the impact of smoking cessation on dust nicotine levels. There have been few studies that reported levels of PBDEs, PCBs, PAHs, or nicotine in residential dust from geographic regions outside of North America or Western Europe. Reports of residential-dust levels from the developing world are particularly sparse. The ease with which residential-dust samples can be obtained creates an opportunity to use dust as a medium for measuring indoor chemical contamination in the developing world. Another possibility for future research could be to identify techniques for the remediation of chemical contamination. For example, PBDE-contaminated e-waste from the U.S. is frequently exported to recycling centers in developing countries, a practice that is neither equitable nor sustainable. Measurement of PBDEs in dust samples before and after remediation would be useful in gauging the impact of these efforts on reducing contaminant levels. Likewise, investigators could use nicotine levels in residential dust to determine the effectiveness of various methods for removing residual tobacco smoke from residences formerly occupied by smokers.Scholars have published extensively on the multifunctional benefits of urban agriculture including: promoting urban sustainability, reducing air and water pollution, building social cohesion, promoting community health and nutrition, teaching food literacy, and creating radical economic spaces for resistance to the capitalist political economy and structural inequities embedded in the “neoliberal city” . Despite growing evidence of these diverse health, education, and environmental benefits of urban agriculture, these vibrant spaces of civic engagement remain undervalued by city policy makers and planners in the United States. Thriving urban farms and gardens are under constant threat of conversion to housing or other competing, higher-value land uses due to rising land values, and other city priorities. This land use challenge and threat to urban farm land tenure is especially characteristic of U.S. cities like San Francisco, one of the most expensive land and housing markets in the country. Under the current urban agriculture paradigm in the U.S., food justice scholars and advocates either try to quantify and highlight the multiple benefits of UA or pursue a critical theoretical approach, arguing that urban agriculture can yield unfavorable results if pursued without an equity lens, especially in cities with intense development pressures and gentrification concerns . A productivist focus is problematic, because, while urban agriculture can be an important component of community food security, its other social and ecological benefits are just as, and sometimes more, significant . In this article, we suggest that the current debates around “urban agriculture” in the U.S. often lead to an unhelpful comparison with rural farms regarding yield, productivity, economic viability, and ability to feed urban populations, most notably in the policy arena.