Sources of bias in ecological studiesof non-rare events

Ecological studies investigate relationships at the level of the group, rather than at the level of the individual. Although such studies are a common design in epidemiology, it is well-known that estimates may be subject to ecological bias. Most discussion of ecological bias has focused on rare disease events, where the tractability of the loglinear model allows some characterization of the nature of different biases. This paper concentrates on non-rare events, where the Poisson approximation to the binomial distribution is not appropriate. We limit the discussion to bias that arises from within-area variability in exposures and confounders. Our aims are to investigate the likely sizes and directions of bias and, where possible, to suggest methods for controlling the bias or for addressing the sensitivity of inference to assumptions on the nature of the bias. We illustrate that for non-rare events it is much more difficult to characterize the direction of bias than in the rare case. A series of simple numerical examples based on a chronic study of respiratory health illustrate the ideas of the paper.     

Reactive nitrogen and human health: acute and long-term implications

Reactive-nitrogen (Nr) has a wide variety of beneficial and detrimental effects on human health. The most important of the beneficial effects are increasing global and regional food supplies and increased nutritional quality of available foods. However, lack of adequate dietary intake of amino acids and proteins is a serious cause of malnutrition when food supplies are inadequate because of poverty, drought, floods, wars, and displacements of people as refugees. There is sufficient, though limited, quantitative data indicating that increased circulation of Nr in the environment is responsible for significant human health effects via other exposure pathways. Nr can lead to harmful health effects from airborne occupational exposures and population-wide indoor and outdoor air pollution exposures to nitrogen dioxide and ozone. Nr can also affect health via water pollution problems, including methemoglobinemia from contaminated ground water, eutrophication causing fish kills and algal blooms that can be toxic to humans, and via global warming. The environmental pollutants stemming from reactive nitrogen are ubiquitous, making it difficult to identify the extent to which Nr exerts a specific health effect. As all populations are susceptible, continued interdisciplinary investigations are needed to determine the extent and nature of the beneficial and harmful effects on human health of nitrogen-related pollutants and their derivatives.     

Effectiveness of radon control techniques in fifteen homes

Radon control systems were installed and evaluated in fourteen homes in the Spokane River Valley/Rathdrum Prairie and in one home in Vancouver, Washington. Because of local soil conditions, subsurface ventilation (SSV) by pressurization was always more effective in these houses than SSV by depressurization in reducing indoor radon levels to below guidelines. Basement overpressurization was successfully applied in five houses with airtight basements where practical-sized fans could develop an overpressure of 1 to 3 Pascals. Crawlspace ventilation was more effective than crawlspace isolation in reducing radon entry from the crawlspace, but had to be used in conjunction with other mitigation techniques, since the houses also had basements. Indoor radon concentrations in two houses with air-to-air heat exchangers (AAHX) were reduced to levels inversely dependent on the new total ventilation rates and were lowered even further in one house where the air distribution system was modified. Sealing penetrations in the below-grade surfaces of substructures was relatively ineffective in controlling radon. Operation of the radon control systems (except for the AAHX's) made no measureable change in ventilation rates or indoor concentrations of other measured pollutants. Installation costs by treated floor area ranged from approximately $4/m2 for sealing to $28/m2 for the AAHX's. Based on the low electric rates for the region, annual operating costs for the active systems were estimated to be approximately $60 to $170.     

Highlights from a new chemical industry report benchmarking performance,policies, and expenditures

Environmental Information Services Inc. (EIS) publishes detailed reports and offers custom research related to the full spectrum of environmental issues facing corporations. This article highlights several key sections from EIS's new 1997 Chemical Industry Report. It details the environmental performance, policies, and expenditures of the largest companies in the chemical industry. EIS industry reports contain data on five key performance criteria—toxic waste management, hazardous waste generation, penalties, spills, and Superfund sites. Here we examine one of those—penalties—and the policies of three of the largest companies in the chemical industry. In the full report, which can be obtained from EIS, 15 corporations are analyzed based on all the performance criteria.     

Laser-induced fluorescence and fast gas chromatography/mass spectrometry with subsurface thermal extraction of organics: Field analytical technologies for expediting site characterization and cleanup

This article describes new field-based technologies that support the Dynamic Workplan/Adaptive Sampling and Analysis Program employed to better characterize hazardous waste sites at lower cost. A laser-induced fluorescence (LIF) probe was designed and field-tested with a cone penetrometer (CP) for investigating petroleum contaminants present at under-ground tank farms. The LIF produced real-time quantitative data for naphthalene and semiquantitative results for total petroleum hydrocarbons and diesel range organics. Two different projects are described employing fast gas chromatography/mass spectrometry (GC/MS) in the field. A 70-foot heated transfer line and probe were used in combination with a cone penetrometer to thermally extract (TE) subsurface soil-bound semivolatile organics. In the second project, soil samples were collected by a Geoprobe^TM and brought to the surface for analysis. The direct measurement TECP data produced semiquantitative results while the more conventional means of collecting and analyzing samples produced risk analysis quality data in the field. A new set of mass spectrometry algorithms provided the technology breakthrough for identifying and quantifying a wide range of Environmental-Protection-Agency-listed target compounds in the presence of high levels of matrix (petroleum) interferents under fast, 7 min/sample, GC conditions.     

Further predictors of renal dysplasia in fetal obstructive uropathy: Bladder pressure and biochemistry of ‘fresh’ urine

Urine was aspirated on two consecutive days from the dilated bladder of nine fetuses with lower urinary tract obstruction. Gestational age ranged from 17 to 35 weeks. Renal dysplasia was diagnosed histologically in four fetuses, whereas the other five had normal renal histology or only partial dysplasia. Urinary sodium (Na⁺) and osmolality (Osm) decreased significantly in the second urine sample 1 day after bladder emptying (median decrease: Na⁺ = −11.3 per cent; Osm= −13.3 per cent). Although there were no significant differences between fetuses with or without renal dysplasia, normalization of an initially raised urine Na ⁺ concentration occurred at the second sample in a fetus with partially normal renal histology, thus correcting a false-positive diagnosis of dysplasia. Bladder pressure was measured at the time of the first urine sampling in seven fetuses and in a further eight with bladder outlet obstruction undergoing a single urine aspiration at 18–28 weeks. Bladder pressure was increased above the reference range in 8 of 15 fetuses with urinary obstruction, but there was no correlation between pressure and the degree of impairment of renal function. Although no conclusive clinical guidelines can be drawn from this study for the evaluation of fetal renal function, these findings suggest that, in lower urinary tract obstruction, tubular reabsorption is impeded by the standing pressure in the urinary tract and that improvement of renal function may occur following relief of obstruction.     

Disappearing lung echogenicity in fetal bronchopulmonary malformations: A reassuring sign?

Congenital bronchopulmonary malformations detectable on prenatal ultrasound include cystic adenomatoid malformation (CAM), lobar sequestration, and upper airway atresia. We describe three fetuses with prenatally detected intrathoracic lesions in which the associated pulmonary hyperechogenicity disappeared before delivery. In the first case of pulmonary sequestration, the infant was asymptomatic after birth. However, in a case of CAM and another with laryngeal atresia, respiratory distress developed after delivery, despite recent scans showing apparently normal lung fields. This experience suggests that ultrasonic resolution of hyperechogenic lung lesions in utero does not necessarily indicate resolution of the underlying pathology.     

Does exaggerated morphology preclude plasticity to cuckoldry in the midshipman fish (<Emphasis Type="Italic">Porichthys notatus</Emphasis>)?

In species with more than one male reproductive morph, there typically exists a larger morph with exaggerated secondary sexual characters, and a smaller morph with reduced secondary sexual characters. These exaggerated and reduced morphologies are commonly thought to represent specializations to alternative behavioral reproductive tactics—large body size and exaggerated secondary sexual characters should both facilitate territoriality, courtship, and pair-spawning; while small body size and reduced secondary sexual characters should facilitate sneaky cuckoldry. Given this postulated relationship between morphology and behavior, we examined the relationship between the morphology of exaggerated males and cuckoldry. In a field and aquarium study of the midshipman fish, a fish with both exaggerated and reduced morphs, we demonstrated cuckoldry in some males of the exaggerated morph. Since the reduced morphology is thought to be an adaptation towards sneaky cuckoldry, we predicted that, of males with the exaggerated morph, less-exaggerated (smaller) males would be better able to gain proximity to the spawning pair during cuckoldry. In contrast to that prediction, access to the spawning pair during cuckoldry increased with the body size of the cuckolding exaggerated-morph males. This may be related to our observation that exaggerated males often cuckolded aggressively. Thus the exaggerated morphology need not preclude adaptive plasticity to cuckoldry, and may even aid it.     

Phenotypic assortment by body shape in wild-caught fish shoals

Phenotypic variation plays a critical role in determining the structural organisation and ecological function of wild populations. Animal groups are often structured according to factors such as species, sex, body size and parasite load, but it is unclear whether body shape also influences patterns of social organisation, and thus contributes to population phenotypic structure. Here, we use geometric morphometric analyses to determine whether wild-caught shoals of a freshwater fish, the western rainbowfish (Melanotaenia australis), are structured according to body size and shape. Using randomisation analyses, we show that the level of variation in size and shape observed in natural group assemblages is lower than that expected under a null model of random shoal composition. In addition, we found evidence of further phenotypic structuring along an upstream-downstream environmental gradient. The putative benefits of morphological assortment include a reduction in predation risk (due to prey oddity and predator confusion effects) and increased hydrodynamic or foraging efficiency. We suggest that morphological variation is a neglected component of population social organisation that can affect population processes, such as patterns of gene flow, and ecological interactions, such as predator-prey dynamics.     

Minimum Infective Dose of the Major Human Respiratory and Enteric Viruses Transmitted Through Food and the Environment

Viruses are a significant cause of morbidity and mortality around the world. Determining the minimum dose of virus particles that can initiate infection, termed the minimum infective dose (MID), is important for the development of risk assessment models in the fields of food and water treatment and the implementation of appropriate infection control strategies in healthcare settings. Both respiratory and enteric viruses can be shed at high titers from infected individuals even when the infection is asymptomatic. Presence of pre-existing antibodies has been shown to affect the infectious dose and to be protective against reinfection for many, but not all viruses. Most respiratory viruses appear to be as infective in humans as in tissue culture. Doses of <1 TCID₅₀ of influenza virus, rhinovirus, and adenovirus were reported to infect 50% of the tested population. Similarly, low doses of the enteric viruses, norovirus, rotavirus, echovirus, poliovirus, and hepatitis A virus, caused infection in at least some of the volunteers tested. A number of factors may influence viruses’ infectivity in experimentally infected human volunteers. These include host and pathogen factors as well as the experimental methodology. As a result, the reported infective doses of human viruses have to be interpreted with caution.