The assessment of health risk due to environmental contaminants depends upon accurate estimates of the distribution of population exposures. Exposure assessment, in turn, requires information on the time people spend in micro-environments and their activities during periods of exposure. This paper describes preliminary results including study methodology and population sampled in a large Canadian survey of time-activity patterns. A 24-hour diary recall survey was performed in 2 381 households (representing a 65% response rate) to describe in detail the timing, location and activity pattern of one household member (the adult or child with the next birthday). Four cities (Toronto, Vancouver, Edmonton and Saint John, NB) and their suburbs were sampled by random-digit dialling over a nine-month period in 1994/1995. Supplemental questionnaires inquiring about sociodemographic information, house and household characteristics and potential exposure to toxins in the air and water were also administered. In general, the results show that respondents spend the majority of their time indoors (88.6%) with smaller proportions of time outdoors (6.1%) and in vehicles (5.3%). Children under the age of 12 spend more time both indoors and outdoors and less time in transit than do adults. The data from this study will be used to define more accurately the exposure of Canadians to a variety of toxins in exposure assessment models and to improve upon the accuracy of risk assessment for a variety of acute and chronic health effects known or suspected to be related to environmental exposures.

Key words:Environmental pollution; exposure assessment; time-activity

Introduction

The Canadian Human Activity Pattern Survey (CHAPS) was undertaken to provide contemporary Canadian data for exposure assessment modelling. In air pollution exposure models, for example, the simplest model uses a time-weighted average of outdoor and indoor exposures. Actual exposure measurements vary greatly from the simple model because people spend different amounts of time in a variety of micro-environments that correspond to different levels of exposure.^1-3 To characterize a population's exposure to a number of environmental agents, data on the population distribution of time spent in specific locations and the range and distribution of time-activity within the population are required. Such exposure assessment is, in turn, an integral feature of any subsequent health risk assessment or related cost-benefit analysis.

While the face validity of direct observation and recording of a subject's activity is high, the costs are also very high, and the presence of the observer may alter the behaviour of the observed. An element of this method is used, however, in recall diaries where a parent or school worker reports on a subject who is too young to recall, record and report activities and locations.

The diary has become the primary method by which time-activity data are collected. A diary can be kept in real time, resulting in the highest number of entries per hour, or by recall, resulting in a lower subject burden but trading off for a lower number of activities and locations remembered.⁴

A modification of the recall diary method was developed and used by the California Air Resources Board.⁵ This is a telephone interview diary using computer-assisted telephone interview (CATI) technology. In this method, the subject's recall is stimulated by prompts to the interviewer from the computer if there are contradictions in reported time-activity patterns or large unvarying time periods reported during the interview. In combination with random-digit dialling techniques, the advantage is that populations that are widely dispersed geographically can be sampled efficiently; the disadvantage of having to rely on the subject's recall or the observation of others remains.

The same technology has subsequently been used to study time-activity patterns in 10,000 subjects throughout the United States by the Environmental Protection Agency ⁶ in the National Human Activity Pattern Survey (NHAPS). The continental US was divided into six regions and randomly sampled over a two-year (eight-season) period in 1993 and 1994. A 24-hour time-activity diary was CATI-administered along with supplemental questionnaires directed at air or water-source pollution exposures. Study results are not yet available from this large survey.

In Canada, less detailed work has been done looking at human activity patterns. A random-sample study of time use, but not location, among Halifax residents was performed by random-day survey in 1971 ⁷ and repeated in the same city in 1981 ⁸ to examine changes over time. Another independent time-use study was conducted in Vancouver in 1971, again concentrating on the activity more than the location.⁹ Finally, the 1992 General Social Survey of Statistics Canada collected information on time use through a time diary. Unfortunately, this information on 9 946 subjects provides only general categories of locations of the respondents; there is not enough event detail for exposure assessments.¹⁰

The purpose of the present study was to collect representative time-activity pattern data on a sample of the population and generate distributions of time-activity patterns for the general population. The same methods and survey team were used in the Canadian study as were used in the EPA study, with minor adjustments in the questionnaire for the Canadian milieu and the addition of questions regarding air conditioning. This paper outlines the methodology used, describes the population surveyed by region, age and sex and provides a general overview of time spent in major activities and locations.

Methods

The survey instrument was composed of three parts. The first was a 24-hour recall diary in which each subject reported their activities as described below. The second part was a set of questions regarding a variety of sociodemographic, family and housing characteristics (such as type of housing, heating and air conditioning). Thirdly, subjects were randomized to one of two supplemental questionnaires, one directed toward determining exposure to airborne pollutants and one directed toward water-borne pollutants.

In the 24-hour recall diary, respondents listed all of the activities they could recall, no matter how brief, for a 24-hour period beginning at midnight the day before the interview. For each activity listed, the interviewer questioned the respondent as to where they were located, the number of minutes spent at that activity and whether they were exposed to tobacco smoke at that time. Fifty percent of the sample, selected at random, were asked if any activity involved elevated breathing rates.

As the subject replied, the data was entered by appropriate activity and location using preset codes. If the subject did not indicate a change in activity over a prolonged period of time, the interviewer probed for any possible oversights. The computer aided the interviewer with chronologically ordering the day's activities even if the subject tended to skip about.

Respondents were asked if they smoked in the preceding 24-hour period, and, if so, how many cigarettes, cigars or pipes they smoked and where they smoked. The only health question per se was a single question that asked about the presence of asthma, chronic bronchitis or emphysema, or angina in the respondent.

Study Population

Data used for this analysis were collected during three periods of the year. From November 8, 1994 to January 8, 1995, a sample of 390 people were interviewed in Toronto and Vancouver. From January 16, 1995 to May 5, 1995, a further 1 239 interviews were conducted in these cities as well as in Edmonton and Saint John (New Brunswick). From June 19 to August 30, 1995, 752 more interviews were conducted in Vancouver, Toronto and Saint John. Urban cities were the only populations sampled. Financial limitations prevented both rural and urban populations being sampled and, of the two, urban areas were chosen because they contained a greater concentration of people.

In each location telephone exchanges were identified in the central city and in the suburban region of the city from telephone company maps. Telephone numbers were selected at random using a two-stage Waksberg-Mitofsky random-digit dialling sample design.¹¹

The primary sampling units (area code + telephone exchange + first two digits of phone number) were randomly assigned to either the weekend or weekday sample, with each day of the week of approximately equal frequency. For example, the weekend sample was called on Sundays and Mondays and consisted of Saturday and Sunday time diaries, respectively. A call-back procedure was used in order to eliminate the problem of non-response and to achieve a high response rate. All non-interview samples were given a minimum of 20 call attempts before finalized as non-completed.

The target population was all persons in households with telephones within the telephone exchanges in the selected cities and suburbs. In households consisting of only adults(respondents aged 18 or older), an adult was selected at random from among all adults residing there using the "next birthday" method (a method that selects among members of a household by inquiring whose birthday is next and selecting that person).

In households with both adults and children (aged 17 or younger), a child was selected at random from among the children living there using the "next birthday" method 60% of the time. Children were oversampled because they are the population most at risk for many contaminants, whether because of developmental immaturity or susceptibility at the time of exposure or because of long lead times between exposure and health outcomes. The other 40% of the time, an adult was selected.

For a respondent under age 10, the adult in the household who was most knowledgeable about the child's activities was asked to complete a proxy interview for the child. For any youth respondent aged 10-17, an adult answered general questions pertaining to the household and some demographic questions; the youth answered the time diary and the post-diary questions about activities on the diary day.

Data Analysis

Results from the three time periods were merged for this analysis, and descriptive statistics were generated using SAS.¹² Weights were applied for the analysis: one for the probability of being called, based on the number of phone lines into the house and the proportion of the overall study population of the cities and suburbs in which the telephone exchanges were selected, and a second weight based on the probability of being a child or adult respondent.

By classifying the 82 locations in which Canadians spend time into 6 major location groups (indoors at home, work/school, indoors-other, bar/restaurant, outdoors, in vehicle), the basic differences in activity patterns between age groups and on weekend/weekdays were examined. The category "indoors-other" included all time spent at grocery stores, shopping malls, hospitals, churches, auto repair shops, hotel/motels, dry cleaners, beauty parlours and a host of other indoor responses. "Bar/restaurant" was treated as a separate category because of the high likelihood of exposure to environmental tobacco smoke in such places. These six groups were adopted to make our results comparable to previous time-activity analyses.^5,6

Results

Table 1 outlines the survey response rates for Canadian households in all four areas along with reasons for non-participation. There were a total of 2 381 completed interviews, a response rate of 64.5%. Some individuals declined to answer individual questions within the interview, such as age or level of education attained.

Approximately 28% of the interviews were weekend data and 72% of the questionnaires were weekday replies. The days of the week were relatively evenly distributed, except for a slight overrepresentation on Tuesdays and a slight underrepresentation on Fridays.

Overall, there were more female respondents (50.9%) than male (49.1%). This female preponderance is more marked in the adults, with males predominating in the children's (55.7%) and youths' (51.7%) interviews. Table 2 outlines the age and sex distribution of respondents, given that 78 adults declined to give their exact ages and 2 declined to state their sex. At the two extremes of age, there were 160 adults over age 70 who answered the questionnaire and 41 proxy interviews for children less than 1 year old. CHAPS respondents over age 25 were more likely than the general population of the same age to have a university degree (29.8% vs 12.8% in 1991 Census ¹³ ).

Among the CHAPS respondents, there were 22 pregnant women, 194 people with asthma, 54 who had angina and 101 who had chronic bronchitis or emphysema. Four hundred and seventy-three (25.9% of adults) reported smoking on the day of the 24-hour interview. This is comparable to the latest Health Canada smoking survey, in which 25% of those over 15 were daily smokers and 5% reported smoking, but not every day.¹⁴

Table 3 illustrates the percentage distribution of time spent (mean +/- standard error) in the major locations for adults, youths and children. The most striking finding is the great deal of time that every age group spends indoors, usually in their own home. By grouping the four indoor categories, we see that respondents spend 88.6% of their time indoors, 65.9% of it in their own homes. Even after accounting for sleeping and napping, which represented about 36% of people's time and was reported overwhelmingly to be in their own homes, this leaves a great deal of waking hours in the home. Children spend even more time indoors (88.8% of their time), than do adults and youth who, in turn, spend more time at work and school. Therefore, on a time-spent basis, the greatest potential for exposure is to possible environmental hazards in the home.

Children and youths spend more time outdoors (7.6% and 8.6%, respectively) than do adults (5.5%). Overall, adults spend more time in vehicles, mainly automobiles. Nevertheless, children spend a reported 3.6% of the time in vehicles, which translates into 50 minutes per day on average.

Figures 1 and 2 demonstrate the differences in activity pattern for all respondents, comparing weekdays and weekends in the six location categories. The dominance of the indoors-at-home location is maintained on both weekdays and weekends; as would be expected, less time is spent at work or school on the weekend. The latter is only partly made up for by time spent outdoors; rather, during the midday more time is spent indoors in other locations than at home and in vehicles than would be spent on a weekday.

In Figure 3, a specific example is drawn from the children's interviews to demonstrate the mean percentage (+/- SE) of time spent outdoors, where other exposures may occur, and its distribution throughout the weekday.

Discussion

The ability to extend the time-activity data to the Canadian population depends upon the representativeness of the sample, which can be assessed by response rates and sociodemographic data. The overall response rate of 64.5% of eligible sampling units is similar to the California study,⁵ where the rate was 61%. The CHAPS study design excluded those without telephones (about 1% of Canadian households)¹⁵ and households of non-English-speaking persons (about 9% of households sampled).

There were, however, only four Canadian cities surveyed and not all seasons were sampled in all areas. Inasmuch as these cities and other cities and provinces may differ by some crucial characteristics or at certain times of the year, the sample may not be representative of daily time-activity patterns of all urban Canadians. The sample also did not contain households from rural areas, and therefore was not representative of the rural population.

The differences in education completed from the general population suggest there may be a tendency to a higher socio-economic status in respondents than in the general Canadian population. The impact that this and other demographic factors have on time-activity will be examined more thoroughly in the larger US study.

When assessing environmental exposure in respondents, it is clear that the majority of time is spent indoors at home. In the California Air Resources Board study, which closely resembles the current study in methods, the proportion of time spent indoors, outdoors and in vehicles was 87%, 6% and 7%, respectively.⁵ This is within two percentage points of our results in each of the three major categories. The effect of indoor exposures, particularly at-home exposures, will greatly influence exposure estimates in environmental risk assessment even when concentrations are low because of the preponderance of time spent indoors.

The finding that children and youths spend greater amounts of time outdoors may be significant in assessing risks related to certain environmental contaminants. Ozone, for example, is an outdoor air pollutant that has much lower concentration indoors.³ Moreover, the concentration peaks in the afternoon in the summer. By examining children's time-activity responses in more detail (as in Figure 3), we see that afternoon coincides with the time they are most likely to be outdoors. Hence, insofar as ozone is a population health risk, it may be more hazardous for children, who have an increased exposure potential that is time-location-specific. This is but one example of how specific exposures may be assessed in subsamples of the population using data from this study.

Acknowledgement

Dr Leech is supported under an Interchange Canada agreement between Health Canada and the University of Ottawa.

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Judy A Leech and K Laporte, Air Quality Health Effects Research, Division of Environmental and Occupational Toxicology, Environmental Health Directorate, Health Canada, Tunney's Pasture, Address Locator: 06083C, Ottawa, Ontario K1A 0L2
K Wilby, Division of Monitoring and Criteria, Environmental Health Directorate, Health Canada
Ed McMullen, Biostatistics, Environmental Health Directorate, Health Canada

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