Adverse pregnancy outcomes near landfill sites in Cumbria, Northwest England, 1950-1993

Adverse pregnancy outcomes near landfill sites in Cumbria, Northwest England, 1950-1993

Trevor J.B. Dummer

DISPOSAL of domestic and industrial waste in landfill sites may expose local residents to dioxins and other chemicals. (1) Such pollutants many of which act as endocrine disrupters–are hazardous to human health, (2,3) although little is known about the effects of low-dose environmental exposures. (4,5)

There have been few epidemiological investigations into the risk of stillbirth and neonatal death associated with living in proximity to landfill sites, especially those sites which deal with household waste. (6,7) In recent studies throughout Europe, investigators have reported an excess risk of congenital anomaly in babies whose mothers lived close to landfill sites used for the disposal of hazardous chemicals. (6,8) Elliott et al. (7) reported a small excess risk of congenital anomaly, but no excess of stillbirth, in babies born within 2 km of municipal landfill sites in the U.K. Studies in North America have shown an increased risk of low birth weight and congenital anomaly in babies born to mothers who lived near hazardous waste sites (9-13) and municipal landfill sites. (14) Other studies have found no evidence of such an association. (15-17) However, all such studies have had difficulties (a) in ascertaining cases of congenital anomaly, inasmuch as there are few high-quality, complete case registers; and (b) in adjusting fully for socioeconomic risk factors at the individual level because of problems in obtaining the necessary information. (7,18)

In the current study, we investigated the risks of stillbirth, neonatal death, and lethal congenital anomaly in the offspring of mothers who resided near landfill sites in Cumbria, northwest England, between 1950 and 1993. We employed a research database that provided very high ascertainment of stillbirths and neonatal deaths (i.e., ascertained through the use of contemporary national registration systems, with additional searches of hospital records where possible) over a 44-yr period, (19) as well as subject-specific data on social class. (20) Adverse outcomes were investigated in regard to proximity to landfill sites that handled industrial wastes, and to general-purpose sites in which domestic waste and other materials were disposed.

Method

The Cumbrian Births Database. Our study area consisted of the county currently defined as Cumbria, in northwest England. (21) The cohort comprised all 283,668 live births and 4,325 stillbirths born to mothers resident in the study area during 1950-1993. (22) For most of the study period, stillbirth was defined as a fetal death occurring after 28 wk gestation. Subsequent to October 1, 1992, however, fetal deaths that occurred after 24 wk gestation were also included, consistent with current legal definitions. (23,24) Death registrations for the cohort–including deaths that occurred outside Cumbria–were supplied by the U.K. Office for National Statistics. Hospital records within Cumbria, and in regional referral centers outside Cumbria, were searched to improve ascertainment of stillbirths and infant deaths. (19)

All causes of stillbirth (data available since 1961) and infant death were coded using International Classification of Diseases, 9th rev. (ICD-9) codes. (25) Causes of death were confirmed, where possible, by examination of medical and/or postmortem records. Neonatal death was defined as death within the first 4 wk of life. A number of outcome groups were defined: stillbirth; neonatal death; stillbirth plus neonatal death; and lethal congenital anomaly (overall and by cause category) identified up until 1993, regardless of age at death. Deaths from congenital anomaly (ICD-9 codes 740-759) were grouped by cause into the following hierarchical and mutually exclusive categories: (a) all neural tube defects (NTDs) (ICD-9 codes 740-742), (b) congenital heart defects (ICD-9 codes 745-747), and (c) other congenital anomalies. NTDs were subdivided into: anencephalus (ICD-9 code 740), spina bifida (ICD-9 code 741), and “Other congenital anomalies of nervous system” (ICD-9 code 742).

The mother’s address on the child’s birth certificate was postcoded (i.e., ZIP-coded) and given a geographical grid reference. (26) The father’s occupation on the birth certificate was assigned a social class on the basis of Office of Population Censuses and Surveys Standard Occupational Classification. (27) Algorithms based on parents’ names, which employed Soundex codes to allowed for variation in the spelling of names, were used to identify siblings, and thereby to assign birth order and identify multiple births.

Geographical data. Since 1976, registration of landfill sites in the U.K. has been a legal requirement. Details of all landfill sites in operation were obtained from the U.K. Environment Agency for the period 1976-1993. For the period prior to 1976, details of landfill sites in Cumbria were obtained by consulting multiple sources, including Cumbria County Council records and the Sitefile Digest. (28) The sites included in our study are shown in Table 1. All sites were grid-referenced, and the time periods of operation ascertained from the Environment Agency. On the basis of Sitefile and Environment Agency (28) classifications, we assigned each site a type code that described the wastes treated, in order from lowest to highest potential toxicity, as follows: Type 1–inert, Type 2–nonhazardous, Type 3–household/putrescible, Type 4–difficult (i.e., difficult to handle). Higher order sites can accept lower order wastes, but not vice versa. For this study, sites of types 1 and 2 were grouped together because only 8 type 2 sites were in operation in Cumbria during the study period. As a result of the U.K. policy of co-disposal of industrial and domestic wastes, type 3 sites may contain materials of a potentially hazardous nature not defined as difficult to handle; (7) further, type 4 sites are not necessarily restricted to hazardous materials, but can accept all liquids and sludges that are difficult to handle. Therefore, we also grouped type 3 and 4 sites together and analyzed them separately. All landfill sites were captured into the Arc/Info geographical information system (GIS) (29) in order to determine distances to the mothers’ residences.

Statistical analysis. No specific pollution data were available on landfill emissions, either by chemical composition or whether released by air or seepage. In addition, we did not have information on site-specific geological, meteorological, or water supply conditions that might affect human exposure to landfill pollutants. Consequently, we relied on a surrogate measure of exposure to landfill pollution based on distance. For each birth, a proximity, P, measure of exposure to landfill sites was computed using the distance function P = 1/(D + 0.1) (2), where D is the distance, in kin, from the site to the mother’s address at the time of the birth. The measure was summed over all sites in operation at the time of birth (0.1 was added to the denominator to ensure that the function remained finite for D = 0; P= 100).

Stillbirth and neonatal death rates declined substantially over the course of the study period, largely as a consequence of improvements in public health and obstetric and perinatal care. (30) Inasmuch as the cause of stillbirth was recorded on the stillbirth registration document only from 1961 onward, we stratified the analysis by time period: 1950-1960, 1961-1975, 1976-1993. The final time period coincided with the formal registration of landfill sites by the Environment Agency. The analysis of lethal congenital anomalies was restricted to the years 1961-1993.

We used multivariate logistic regression (31) to model the risk of each outcome by proximity to landfill sites. Adjustment was made for known demographic risk factors (i.e., year of birth [modeled using a linear and quadratic term for year], social class, birth order, and multiple births) by using offsets from an analysis of the effects of demographic risk factors without the distance function. This allowed a more robust estimate of these effects, avoiding difficulties in effect estimates as a result of confounding by, for example, socioeconomic status and proximity to landfill sites. The use of offsets in covariate adjustment has been discussed extensively elsewhere. (32) The analysis was stratified by time period and site type (types 1 + 2, type 3, type 4, types 3 + 4). Adjusted continuous odds ratios (ORs) for risk of adverse outcome in relation to proximity to landfill sites are presented for specific values of the exposure function. Because multiple births (which comprise fewer than 1% of births) may not be independent events, robust estimates of variance based on the Huber/White sandwich estimator of variance were used, and statistical significance was assessed from the corresponding confidence interval (CI). (33) These variance estimates tended to be slightly conservative, but in practice were little different than the naive estimates. For any statistically significant results, a sensitivity analysis was carried out that repeated the logistic regression but excluded data on births with the greatest influence, as measured by Pregibon’s influence statistic. (31)

Results

Table 2 shows adjusted ORs for risk of each adverse outcome in relation to proximity to landfill sites, by type of site and by time period. ORs greater than 1 indicate a risk that increases with proximity to landfill sites, whereas ORs less than 1 indicate a risk that decreases closer to landfills.

In most instances, the ORs were around 1, which indicated that there was no association, in this dataset, between increased risk of stillbirth, neonatal death, or lethal congenital anomaly (overall) and residential proximity to landfill sites. The robust 95% CIs showed no evidence of any significant association for stillbirth, neonatal death, or lethal congenital anomaly overall.

For the period 1976-1993, we noted a significantly increased risk for “Other congenital anomalies of nervous system” in relation to proximity to type 3 (household waste) landfill sites (OR = 1.14; 95% CI = 1.03, 1.25), but no increased risk for all NTDs. Because no type 3 sites were in operation between 1961 and 1969, no analytical results are presented for the time period 1961-1975 for these sites. For the entire time period that type 3 sites operated (1970-1993), results were similar. The significantly increased risk was robust to the exclusion of the most influential observations in the dataset. With one exception, there were no significantly increased risks of lethal congenital anomaly related to proximity to landfill sites for any other cause group (results not shown). The exception was for congenital anomalies other than NTDs or heart defects in relation to Type 4 landfill sites, for which there was an association of borderline significance (OR = 1.29; 95% CI = 1.00, 1.66). Covariate adjusted and unadjusted results were similar.

Discussion

During the period 1950-1993 in Cumbria county, northwest U.K., we found no increased risk of stillbirth, lethal congenital anomaly overall, or neonatal death with proximity to landfill sites, within the defined time periods or for different types of landfill sites. However, we did note a small but significantly increased risk of death from “Other congenital anomalies of nervous system” for births to mothers who resided near household waste landfill sites from 1970 onward.

This study had a number of strengths. A cohort study may be less susceptible to selection bias than a case-control study. Our study covered 44 yr–a considerable period of time which allowed us to investigate the risk of stillbirth, lethal congenital anomaly, or neonatal death among more than 250,000 births, by proximity to a potential environmental hazard. Because the Cumbrian Births Database recorded all birth registrations in Cumbria during the study period by date of birth and postal code of mother’s residence, we had precise data on the population at risk and the location of each birth. Consequently, we were able to estimate exposure and risk with a continuous exposure function model, unconstrained by the availability of population statistics from other sources. Unlike other studies, (6-8) we avoided the construction of exposed and unexposed zones (such as concentric circles around landfill sites), which inevitably introduce misclassification of exposure, especially at zone boundaries, and assume homogeneity of risk within the arbitrarily defined zones. Instead, we incorporated proximity of each birth to multiple sites. Because we had demographic information for each birth, we were able to take into account individual risk factors such as socioeconomic status (social class). We have shown previously that socioeconomic status is a better predictor of stillbirth rates than are the community-based deprivation measures (34) used in other studies. (7) Adjustment for individual social class avoided the potentially serious problem of confounding between socioeconomic status and residence near landfill sites. We were able to discriminate between landfill sites on the basis of the type of waste they contained, and we were rigorous in our ascertainment of deaths, stillbirths, and lethal congenital anomalies. (19)

A limitation of our study was the exclusion of non lethal congenital anomalies, which were included in previous studies. (6-8) A further limitation was that, because there were no data on actual pollution levels around each site, we relied on a function of distance as a surrogate for potential exposure. Thus, there existed the possibility of misclassification of exposure to landfill sites. We relied on the U.K. standard classification of landfill site types, which is based on the nature of the materials handled. Because of the long time-scale of our study and the large number of sites incorporated, specific details regarding the quantity and composition of materials processed at each site were unavailable; therefore, we were unable to identify specific environmental pollutants within the landfill site categories.

The form of the proximity function, 1/(D + 0.1) (2), assumed that exposure increased rapidly with proximity to the sites. Other studies have made similar assumptions. Dolk et al. (6) defined a 3-km zone around landfill sites and designated this as an “exposure zone.” Elliott et al. (7) constructed 2-km zones, and other studies have used exposure zones as close as 1.6 km (1 ml) to the landfill site. (11,17) We did not rely on the construction of arbitrary exposure zones, but rather estimated proximity, taking into account all landfill sites. However, the function could not account for wind direction or the flow direction of municipal water sources–data that were unavailable over the time period of this study.

Ascertainment of landfill sites post-1976 was complete. However, prior to 1976 registration of landfill sites was not comprehensive, although we used multiple sources to identify sites in operation during that period. As a result, we may not have ascertained all the early landfill sites, and some proximity measures might have been misclassified for this time period. We found that 85% of the babies born between 1976-1993–the time period for which ascertainment of sites was complete–were born within 2 km of a landfill site. This is consistent with the results of Elliott et al., (7) who found that 80% of the U.K. population live within 2 km of a landfill site.

There was little research available investigating the risk of stillbirth or neonatal death in relation to proximity to landfill sites with which to compare our results. Elliott et al., (7) whose ascertainment of stillbirths was comparable to ours, also found no association between the risk of stillbirth and proximity to landfill sites. (7)

Our findings of an increased risk of death from “Other congenital anomalies of nervous system” are also consistent with the results of Elliott et al., (7) who found an increased risk of all NTDs closer to landfill sites in the U.K. A significantly increased risk of NTDs has also been reported closer to hazardous waste landfill sites throughout Europe. (6) However, Elliott et al. reported an increased risk of NTDs closer to both household (type 3) and difficult (type 4) landfill sites, whereas our results show significantly increased risk being restricted to type 3 sites. This could be because of limited statistical power in our study, or the fact that, unlike Elliott et al., we restricted our analysis to lethal congenital anomaly and used a different method for estimating exposure to landfill site pollutants on the basis of a continuous distance assessment model. The discrepancy between our results for type 3 and type 4 sites might be related to differences in the nature of the materials deposited at these sites. In the U.K., domestic-waste landfill sites (:an be as hazardous environmentally as hazardous-waste landfill sites, particularly because domestic waste sites often process industrial waste as part of the co-disposal policy. (7) Type 4 landfill sites deal with wastes that are difficult to handle, but not necessarily hazardous.

Caution must be used in interpreting the statistical association between proximity to household landfill sites and an increased risk of death from “Other congenital anomalies of nervous system.” In this study, we undertook a number of statistical analyses–stratified by type of landfill site, time period, and adverse pregnancy outcome. Thus there is potential for apparently significant results to arise by chance because of multiple significance testing. Although we found an elevated risk of “Other congenital anomalies of nervous system” for the time period 1970-1993 in relation to household waste landfill sites, we did not find a significantly increased risk of all NTDs over the same period. In addition, we found no significantly increased risk of adverse pregnancy outcomes in relation to Type 4 (difficult-to-handle waste) landfill sites. The significant increased risk of “Other congenital anomalies of nervous system” closer to type 3 sites was consistent across time periods and was robust to the exclusion of the most influential births.

Some of our results from the analysis of lethal congenital anomalies differ from those of other investigators, who have found an increased risk of congenital anomaly near hazardous and household waste landfill sites throughout Europe, (6,8) in the U.K., (7) and in North America. (10,11,12) There are several possible reasons for this. As noted previously, we relied on a surrogate measure of exposure to landfill sites that was based on distance. Within the constraints of the available data–including a lack of geological, meteorological, or water supply information–there is potential for misclassification of exposure. However, most other researchers have relied on surrogate measures of exposure to landfill pollutants because pollution data were lacking. In our study we considered only lethal congenital anomalies, whereas other studies have included nonlethal cases. (6-8) Within the framework of our study, ascertainment of lethal congenital anomaly cases was very high. Some other studies have relied on U.K. national registration data on congenital anomalies, (7) which is acknowledged to be incomplete, and which may be differential with respect to proximity to landfill sites. (18,35) By careful scrutiny of original records from a variety of sources, we were able to construct a comprehensive list of landfill sites that accepted domestic and industrial wastes–including more than 80 sites that dealt with potentially hazardous domestic and industrial material and which were not included in other studies (6,8)–and we considered these within a cohort study, thereby avoiding the limitations of a case-control design.

Conclusions

We found no evidence to suggest all increased risk of stillbirth or neonatal death with proximity to landfill sites in Curnbria during 1950-1993. However, we did find a small but statistically significant increased risk of death from “Other congenital anomalies of nervous system” (ICD-9, (25) code 742) associated with household and putrescible waste landfill sites, for the period from 1970 onward. Because we had no data on pollution levels by either quantity or type around the landfill sites, and no detailed information on geological, water supply, or meteorological conditions that could affect exposure pathways, we could not infer a casual effect from this statistical association. We suggest caution in the interpretation of these findings, given the large number of statistical analyses undertaken. However, a statistically significant association between an increased risk of congenital anomaly (including NTDs) and proximity to household waste and hazardous waste landfill sites has been reported on the basis of other datasets in the U.K. and Europe. (6-8) Further investigations will be necessary to establish whether these statistical associations reflect a causal environmental effect. Such work should include the study of both lethal and nonlethal outcomes in relation to actual pollution levels around landfill sites, and should incorporate information on exposure pathways and individual exposures. Because many landfill sites within the U.K. process a combination of household, industrial, and hazardous wastes, it remains difficult to differentiate levels of potential environmental health risk from such sites without direct exposure assessment.

Table 1.–Number of Landfill Sites Studied, by Type and Time Period,

1950-1993

Time period

Waste material

Site type * processed 1950-1960 1961-1975 1976-1993

1 and 2 Inert and

nonhazardous 5 4 105

3 Household/putrescible 0 2 29

4 Difficult-to-handle 5 14 62

* Based on Sitefile Digest and U.K. Environment Agency

classification. (28)

Table 2.–Continuous Odds Ratios (ORs)* for Risk of Stillbirth,

Neonatal Death, and Lethal Congenital Anomaly in Relation to Proximity

to Landfill Sites, by Site Type, Adjusted for Year of Birth, Social

Class, Birth Order, and Occurrence of Multiple Births, 1950-1993

Type 1 and 2 sites

No. of

Outcome cases OR 95% CI

1950-1960

Stillbirth + neonatal death 3,366 0.94 0.83, 1.06

Stillbirth 1,978 0.98 0.88, 1.09

Neonatal death 1,388 0.83 0.63, 1.08

1961-1975 ([dagger])

Stillbirth + neonatal death 3,103 0.96 0.91, 1.02

Stillbirth 1,680 0.85 0.66, 1.09

Neonatal death 1,423 1.02 0.97, 1.06

Congenital anomaly 1,114 1.00 0.93, 1.07

Neural tube defect 580 1.01 0.93, 1.11

Anencephalus 255 1.06 0.98, 1.14

Spina bifida 233 0.89 0.59, 1.35

“Other congenital anomalies

of nervous system” ([double dagger]) 92 0.90 0.45, 1.81

1976-1993

Stillbirth + neonatal death 1,286 1.00 0.96, 1.04

Stillbirth 667 1.01 0.98, 1.05

Neonatal death 619 0.95 0.88, 1.03

Congenital anomaly 455 0.98 0.93, 1.04

Neural tube defect 154 0.80 0.64, 1.00

Anencephalus 40 0.95 0.78, 1.16

Spina bifida 71 0.82 0.58, 1.16

“Other congenital anomalies

of nervous system” ([double dagger]) 43 0.62 0.37, 1.04

Type 3 sites

Outcome OR 95% CI

1950-1960

Stillbirth + neonatal death — —

Stillbirth — —

Neonatal death — —

1961-1975

([dagger])

Stillbirth + neonatal death — —

Stillbirth — —

Neonatal death — —

Congenital anomaly — —

Neural tube defect — —

Anencephalus — —

Spina bifida — —

“Other congenital anomalies

of nervous system” ([double dagger]) — —

1976-1993

Stillbirth + neonatal death 0.99 0.90, 1.08

Stillbirth 1.01 0.91, 1.12

Neonatal death 0.95 0.83, 1.09

Congenital anomaly 1.04 0.94, 1.14

Neural tube defect 1.00 0.79, 1.31

Anencephalus 0.16 0.01, 3.86

Spina bifida 0.32 0.07, 1.57

“Other congenital anomalies

of nervous system” ([double dagger]) 1.14 1.03, 1.25

Type 4 sites

Outcome OR 95% CI

1950-1960

Stillbirth + neonatal death 0.73 0.47, 1.12

Stillbirth 0.57 0.30, 1.09

Neonatal death 0.92 0.53, 1.59

1961-1975

([dagger])

Stillbirth + neonatal death 0.97 0.80, 1.17

Stillbirth 0.97 0.76, 1.23

Neonatal death 0.98 0.73, 1.31

Congenital anomaly 0.92 0.65, 1.31

Neural tube defect 0.55 0.24, 1.26

Anencephalus 0.84 0.40, 1.75

Spina bifida 0.38 0.07, 2.23

“Other congenital anomalies

of nervous system” ([double dagger]) 0.01 0.00, 2.11

1976-1993

Stillbirth + neonatal death 0.99 0.93, 1.06

Stillbirth 1.01 0.93, 1.09

Neonatal death 0.97 0.89, 1.07

Congenital anomaly 1.00 0.93, 1.08

Neural tube defect 0.96 0.82, 1.11

Anencephalus 0.98 0.78, 1.22

Spina bifida 0.84 0.61, 1.66

“Other congenital anomalies

of nervous system” ([double dagger]) 1.05 0.90, 1.23

Type 3 and 4 sites

Outcome OR 95% CI

1950-1960

Stillbirth + neonatal death — —

Stillbirth — —

Neonatal death — —

1961-1975

([dagger])

Stillbirth + neonatal death — —

Stillbirth — —

Neonatal death — —

Congenital anomaly — —

Neural tube defect — —

Anencephalus — —

Spina bifida — —

“Other congenital anomalies

of nervous system” ([double dagger]) — —

1976-1993

Stillbirth + neonatal death 0.99 0.94, 1.04

Stillbirth 1.01 0.95, 1.07

Neonatal death 0.97 0.90, 1.04

Congenital anomaly 1.02 0.95, 1.08

Neural tube defect 0.97 0.84, 1.12

Anencephalus 0.92 0.71, 1.18

Spina bifida 0.79 0.57, 1.09

“Other congenital anomalies

of nervous system” ([double dagger]) 1.09 0.99, 1.21

Note: CI = confidence interval.

* ORs are continuous (e.g., the odds of stillbirth at a distance, D,

from type 2 and 3 landfill sites, compared with the odds at 10 km from

type 2 and 3 sites during 1950-1960, is [0.98.sup.(1/[(D + 0.1).sup.2]

– 1/[10.1).sup.2]]). Therefore, the OR comparing risk at a distance of

0.5 km with that at 10 km for type 2 and 3 sites during 1950-1960 is

approximately 0.95.

([dagger]) Analysis omitted because no type 3 sites were in operation

prior to 1970.

([double dagger]) From International Classification of Diseases, 9th

rev. (ICD-9), (25) code 742.

We are grateful to Newcastle Hospital’s Special Trustees for funding this project, and to the North of England Children’s Cancer Research Fund for ongoing support. We also thank Mr. Julian Smith and Mrs. Jane Salotti for assistance with the Cumbrian Births Database, and Mrs. Katharine Kirton for secretarial assistance.

Submitted for publication June 14, 2002; revised; accepted for publication August 4, 2003.

Requests for reprints should be sent to Prof. Louise Parker, Sir James Spence Institute of Child Health, University of Newcastle, The Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP, U.K.

E-mail: louise.parker@ncl.ac.u k

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TREVOR J. B. DUMMER

School of Social Science

Liverpool John Moores University

Liverpool, United Kingdom

HEATHER O. DICKINSON

Centre for Health Services Research

University of Newcastle

Newcastle, United Kingdom

LOUISE PARKER

Paediatric and Lifecourse Epidemiology Research Group

School of Clinical Medical Sciences

University of Newcastle

Sir James Spence Institute

Royal Victoria Infirmary

Newcastle, United Kingdom

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