There is little doubt that inadequate hygiene practices increase the risk of infection. Infectious diseases prior to the introduction of antibiotics were the single highest cause of mortality in the general population. With the introduction of antibiotics, improvements in sanitation, and the promotion of effective hygiene practices in the home, these fatal infectious diseases have all but been eradicated. In contrast, however, diseases of the immune system, including asthma and atopy,
prevalent over the past few decades. Consequently, it has been suggested that improved hygiene practices in the West may be partly responsible for the increased prevalence of asthma and atopy. The “hygiene hypothesis” proposes that exposure to infectious agents in childhood stimulates the immune system in favor of a Th1 response which inhibits the production of Th2 cells associated with increased IgE, eosinophilia, atopy, and airway hyper-responsiveness. The question arises whether certain hygiene practices inhibit exposure to infectious agents, which in turn would promote the production of allergy-inducing Th2 cells.
The putative association between hygiene and atopy may be confounded and/or mediated by a number of factors, not yet accounted for in many of the published studies. Little is known about hygiene practices in the home or norms of cleanliness in the United Kingdom today and virtually no information is available on the social, demographic, or lifestyle factors influencing hygiene practices.
This paper draws on unique data gathered in the Avon Longitudinal Study of Parents and Children (ALSPAC)8 to provide cleanliness norms for infants aged 15 months. In addition, data gathered prospectively from as early in pregnancy as possible may identify social, demographic, and lifestyle factors influencing hygiene practices within the home. We attempt to analyze as many relevant factors as possible, ranging from factors that may directly affect hygiene levels, to those that may have an influence on the adoption of the particular practice, to those that may have a mediating, confounding, or interacting effect with hygiene in its putative association with atopic diseases.
METHODS
Subjects
The Avon Longitudinal Study of Parents and Children (ALSPAC), formerly known as the Avon Longitudinal Study of Pregnancy and Childhood, is a prospective study of 14 541 pregnancies. Women were enrolled as early in pregnancy as possible on the basis of an expected date of delivery between 1 April 1991 and 31 December 1992, and place of residence within the three Bristol-based health districts of the former county of Avon, UK. It was estimated between 85% and 90% of eligible mothers were enrolled in the study. Of the 14 541 pregnancies enrolled, 13 988 children survived one year.
Data were collected using a variety of sources, including questionnaires completed by the parents, medical records, biological samples, and hands-on examination of the whole cohort from 7 years of age at a research clinic. The ALSPAC website contains detailed information on the study design, data gathered, clinic measurements, and questionnaire response rates
Hygiene score
When their child was 15 months of age, parents were asked how often on a normal day their child’s face and hands were wiped, and hands wiped before meals. Responses ranged from “not at all” to “five or more times per day”. In addition, parents were asked how often their child was given a bath or shower, with responses ranging from “hardly ever” to “more than once a day”. From these responses, a simple cumulative infant hygiene score was derived (ranging from least hygienic to most hygienic).
Putative influencing factors
Possible influential factors of hygiene practices in the home were taken from self-report questionnaires returned by the study mothers from early pregnancy throughout the first year of life of the study child. For some factors, there were multiple time points that the particular question was asked. In these cases, an effort was made to use data collected at the same time, or as close as possible prior to the time the hygiene questions were asked.
Perinatal factors
Birth weight, gestational age, and gender were abstracted from obstetric records. Maternal parity, defined as the number of previous pregnancies resulting in a live birth or fetal death after 28 weeks gestation (0, 1, 2+), was ascertained from a questionnaire administered to the study mothers during pregnancy (at 32 weeks gestation); ethnicity of the study child was determined at 32 weeks gestation. When her baby was 6 months old, each mother was asked about her infant’s mode of feeding in the previous six months. From this, the duration of breastfeeding up to and including 6 months of age was determined.
Maternal factors
Maternal smoking during pregnancy in the final trimester and highest maternal educational achievement were ascertained from maternal questionnaires completed during pregnancy (32 weeks gestation). Maternal age at delivery was obtained from obstetric records and was categorized into three groups (<20 years, 20–24 years, and 25+ years). Maternal paracetamol use in the final trimester of pregnancy was obtained from the 32-week questionnaire; responses were recorded as never, sometimes, most days/every day. A history of asthma/eczema in the mother of the study child was ascertained at 12 weeks postnatally.
Social factors
Housing tenure (mortgaged/owned, local authority rented, other), damp, mold, or condensation in the home, and reports of financial difficulties were obtained from a questionnaire sent out when the study child was 8 months of age.
Environmental factors
When the children were 15 months of age, exposure to environmental tobacco smoke (ETS) at the weekend was determined. The responses were categorized as: never, <1 hour, 1–2 hours, 3–5 hours, and always. Also at 15 months, mothers were asked which of the following pets the study child had been in contact with on at least one occasion during the week, either in the home or elsewhere: cats, dogs, other furry pets, or other non-furry pets. At regular intervals, postpartum, study mothers were asked about the frequency and use of household chemicals such as disinfectant, bleach, aerosols, etc. A score was derived that reflected frequency of use of these chemicals in the home at 8 months of age.
The month in which the 15 months questionnaire was returned was also analyzed according to hygiene score to account for any seasonal changes in hygiene practices.
Statistical analysis
All data were analyzed using SPSS for Windows (version 9.0.0). The hygiene scores were approximately normally distributed Quintiles of the hygiene scores were derived for categorical descriptive analyses. The hygiene score was dichotomized, with scores above 10 (top quintile) representing the group with high hygiene scores and scores of 10 or below representing the reference group. Unadjusted analyses using logistic regression models were carried out for all factors. Multivariable logistic regression models using the backward method of elimination assessed which factors were independently associated with a high hygiene score. The modeling was carried out in three stages. Factors were analyzed univariably in the first instance. Next, the factors within each of the perinatal, maternal, social, and environmental groups were analyzed simultaneously (within groups). All significant factors from the previous stage were then combined in a final model (between groups) to assess independent predictors of high hygiene scores. Statistical significance was based on a 5% significance level using the likelihood ratio statistic.
