Children with Asthma Are Less Likely to Walk to School

Abstract

Determine if children with asthma are more/less likely to walk to school, and if the living distance modifies this association. Five thousand six hundred nineteen children aged 5–9 completed the Toronto Child Health Evaluation Questionnaire. The mode of transportation was defined as walking, driven by car, or school bus. Children were categorized by the International Study of Asthma and Allergies in Childhood methodology as doctor-diagnosed current or lifetime asthma, symptomatic controls (asthma symptoms without diagnosis), or asymptomatic controls. The living distance was the shortest driving distance between home and school. Adjusted models revealed children with current asthma [odds ratio (OR) 1.29; 95% CI 1.01–1.65], lifetime asthma (OR 1.26; 95% CI 1.01–1.56), and symptomatic controls (OR 1.34; 95% CI 1.14–1.59) were more likely to be driven by car versus asymptomatic controls. Among children who lived within walking distance of the school (≤1.5 km), symptomatic controls were more likely to be driven by car (OR 1.30, 95% CI 1.09–1.56) or school bus (OR 1.48, 95% CI 1.01–2.17); however, children with current (OR 1.71, 95% CI 1.00–2.92), or lifetime asthma (OR 1.71, 95% CI 1.06–2.76) were more likely to be driven only by school bus. Children with asthma or respiratory symptoms are less likely to walk and more likely to be driven to school. Among children who are ineligible for school bus programs based on the living distance from school, children with asthma are still more likely to be bussed.

Introduction

Well-controlled asthma should be achievable in the majority of the childhood asthma population with the use of environmental modifications and low and safe doses of anti-inflammatory medications.¹ National and international asthma guidelines stress that children with well-controlled asthma should have no physical or exercise limitations.^2–4

Among children, physical activity contributes to cardiovascular and bone health, psychosocial well-being, and the battle against obesity⁵; it improves asthma symptoms and lung function in children with asthma,^6–9 and improves asthma control.¹⁰ One simple, low-cost form of daily physical activity is active travel, such as walking or cycling to school. Unfortunately, few children are actively commuting to school¹¹ regardless of asthma status.¹² Rates of active travel have declined over the last 50 years in western countries.^13,14 Being driven to school is associated with a higher income, having access to a car or school bus,¹⁵ the travel distance,¹² and parental concerns about safety, including neighborhood violence, traffic volume, and speed.¹⁶ However, associations between different modes of transportation and asthma status have not been previously examined.

Toronto is a multiethnic urban center with a population of 2.5 million and a large new immigrant population (50% foreign born in 2006).¹⁷ The goal of our study was to determine if grade 1 and 2 children with asthma, attending publicly funded schools in Toronto, are more or less likely to walk to school, compared to children without asthma, and if the living distance from the school modifies this association.

Materials and Methods

Sample design and population

The study population was taken from the 2006 Toronto Child Health Evaluation Questionnaire study (T-CHEQ), a multistage cross-sectional study that collected population-based data on asthma, allergy, and related symptoms in 5,619 grade 1 and 2 children (aged 5–9), attending 231 randomly selected publicly funded schools in the city of Toronto. Details regarding the study design and population characteristics have been previously reported.¹⁸ The T-CHEQ is considered to be representative of the population of grade 1 and 2 Toronto school children in 2006, based on sex, asthma status, environmental tobacco smoke (ETS) exposure, dwelling type, household size, and income adequacy. A follow-up study was conducted in 2010 on a subset of these children as part of a nested case–control study.

Exposure variables

Primary exposures were evaluated using the International Study of Asthma and Allergies in Childhood (ISAAC) methodology.¹⁹ Doctor-diagnosed lifetime asthma was defined by positive responses to “Has your child ever had asthma?” and “Was this diagnosed by a doctor?” Current asthma was defined as lifetime doctor-diagnosed asthma with wheeze or medication use for asthma or wheeze in the past 12 months. Henceforth, children free of all respiratory symptoms and asthma diagnoses will be referred to as asymptomatic controls. Children without doctor-diagnosed asthma, but who reported ever having respiratory symptoms (wheeze or nighttime cough not associated with a cold or chest infection) were categorized as symptomatic controls, since many of these children would be expected to have undiagnosed asthma.^20,21

Outcome variable

The primary outcome was the mode of transportation to school. Respondents were classified into one of the following: (1) walks, (2) driven by car, or (3) driven by school bus. Participants who did not specify their mode of transportation or who reported a combination of modes of transportation were excluded from the analyses (n=155). This information was collected by a questionnaire in 2006 and 2010.

Potential confounders

Information on potential confounders, including age, sex, height, weight, income adequacy, history of parental asthma, current ETS exposure, and whether the child was born in Canada, was collected by a questionnaire in 2006. Income adequacy is based on a variable defined by Statistics Canada and describes the total household income adjusted for the number of household members.²² History of parental asthma was determined by a report of maternal asthma, paternal asthma, or both. ETS exposure was determined by a report of anyone smoking inside the child's home. The weight for age was calculated using the Center for Disease Control macro to generate the anthropometric status of children.²³ A child's exposure to ambient traffic-related air pollution (TRAP) was estimated by land use regression and inverse distance weighting using the current child's home address and previously established methods.²⁴

The living distance from school was calculated as the shortest driving distance between home and school addresses using CDXZipstream™(CDX Technologies, Randolph, NJ), which employed Microsoft MapPoint^® software (Microsoft, Redmon, WA). Toronto school board policies state that children who live within 1.5 km of their school are within reasonable walking distance and do not qualify for board-provided transportation unless there are safety concerns or physician documentation indicating the inability to walk due to ongoing medical issues.^25,26 The living distance is therefore defined in the following ways: (1) as a continuous variable (km); (2) dichotomized to ≤1.5 km or >1.5 km from school. The living distance was also examined as an effect modifier. Children who reported walking, but whose residence was >3.2 km from their school were excluded from the analysis (n=56) due to the assumed high risk of misclassification. Toronto school board policies set 3.2 km as a reasonable walking distance for children in grades 5–8; hence, it is highly likely that these children were being driven to a home or daycare that is closer to their school and walking the rest of the way.

Analyses

Potential confounders were systematically evaluated for each outcome. Multinomial logistic regression was used to estimate the association between asthma and the mode of transportation. Bivariate analyses were conducted for all potential confounding variables against the mode of transportation. Variables that showed a statistically significant association (p<0.05) with the outcome were added to the multivariate model. A correlation analysis ensured that no 2 highly correlated variables were entered into the same model. Individual models were developed for each exposure/outcome pair. Final regression models contained variables that maintained their statistical significance, as well as age and sex, which were forced into the multivariate model. Interaction terms for living within or beyond the walking distance of school were added to the models to test for effect modification. If an interaction term was significant, a stratified multivariate analysis was conducted. To determine if the mode of transportation was associated with age, the McNemar's test was used to compare the proportion of children walking in 2006 to the proportion walking in 2010 among the subset of children who completed the follow-up study. All analyses were conducted using SAS statistical software version 9.3 (SAS Institute, Cary, NC).

Results

Our final sample was made up of 5,342 children. Of these, 49.8% were male and the mean age was 6.74 years. Doctor-diagnosed lifetime asthma was reported by 15.5%, current asthma by 11.8%, 29.8% were classified as symptomatic controls, and 52.8% as asymptomatic controls. Where the living distance could be measured (n=5,131), the median distance was 0.76 km. The mode of transportation to school was reported by 5,342 respondents: 54.5% of children reported walking to school, 35.0% reported being driven by car, and 10.6% being driven by school bus (Table 1). Compared to asymptomatic controls, proportionally fewer children with asthma and symptomatic controls reported walking to school and a greater proportion reported being driven by car or by school bus (Table 2). The sample was then stratified by the living distance to school. Among those who lived within walking distance of their school, compared to asymptomatic controls, fewer symptomatic controls and children with asthma reported walking to school, but in this instance, while a greater proportion of symptomatic controls were driven either by car or school bus, a statistically significant difference was only seen with being driven by school bus among children with asthma.

Table 1.

Descriptive Statistics of School Children Aged 5–9 Participating in the Current Study (N=5,342)

Variable	Mean	Median	Standard deviation	Min	Max
Age (n=4,555)	6.73	7.00	0.67	5	9
Distance to school (km) (n=5,131)	1.27	0.76	2.15	0^a	34.78
Walk	0.61	0.55	0.41	0^a	3.16
Driven by car	1.94	1.04	3.15	0^a	34.78
Driven by school bus	2.44	1.83	2.22	0.22	28.78

Variable	n (%)
Male sex (n=5,307)^b	2,642 (49.8)
Current asthma (n=4,865)^b	572 (11.8)
Lifetime asthma (n=5,078)^b	785 (15.5)
Symptomatic controls (n=5,078)	1,511 (29.8)
Asymptomatic controls (n=5,078)	2,782 (52.8)
Distance to school <1.5 km (n=5,131)	4,122 (80.3)
Transportation to school, 2006 (n=5,342)
Walk	2,909 (54.5)
Driven by car	1,869 (35.0)
Driven by school bus	564 (10.6)

Frequencies for each variable are adjusted for missing values.

Microsoft MapPoint^® software is accurate up to 10 m. For any 2 addresses, the software detects as being less than 10 m apart, the driving distance is estimated to be 0.

Reported in a previous publication.¹⁵

Table 2.

Frequency of Mode of Transportation to School by Asthma Status in Total Study Population and Stratified by Living Distance from School in Toronto School Children Aged 5–9 in 2006

	Total population			Children living within walking distance ^a of school			Children living further than walking distance ^a from school
Current asthma	Yes (n=572)	No^b (n=2,782)	P value	Yes (n=437)	No^b (n=2,185)	P value	Yes (n=119)	No^b (n=486)	P value
Walk (n=2,909)	290 (50.7)	1,587 (57.1)	0.01	273 (62.5)	1,478 (67.6)	0.04	10 (8.40)	49 (10.1)	0.58
Driven by car (n=1,869)	219 (38.3)	936 (33.6)	0.03	138 (31.6)	625 (28.6)	0.21	73 (61.3)	268 (55.1)	0.22
Driven by school bus (n=564)	63 (11.0)	259 (9.3)	0.21	26 (6.0)	82 (3.8)	0.03	36 (30.3)	169 (34.8)	0.35

Lifetime asthma	Yes (n=785)	No^b (n=2,782)	P value	Yes (n=604)	No^b (n=2,185)	P value	Yes (n=156)	No^b (n=486)	P value
Walk (n=2,909)	411 (52.4)	1,587 (57.1)	0.02	380 (62.9)	1,478 (67.6)	0.03	17 (10.9)	49 (10.1)	0.77
Driven by car (n=1,869)	293 (37.3)	936 (33.6)	0.06	190 (31.5)	625 (28.6)	0.17	94 (60.3)	268 (55.1)	0.26
Driven by school bus (n=564)	81 (10.3)	259 (9.3)	0.40	34 (5.6)	82 (3.8)	0.04	45 (28.9)	169 (34.8)	0.17

Symptomatic controls	Yes (n=1,511)	No^b (n=2,782)	P value	Yes (n=1,121)	No^b (n=2,185)	P value	Yes (n=331)	No^b (n=486)	P value
Walk (n=2,909)	751 (49.7)	1,587 (57.1)	<0.001	695 (62.0)	1,478 (67.6)	<0.01	25 (7.6)	49 (10.1)	0.22
Driven by car (n=1,869)	569 (37.7)	936 (33.6)	<0.01	362 (32.3)	625 (28.6)	0.03	185 (55.9)	268 (55.1)	0.83
Driven by school bus (n=564)	191 (12.6)	259 (9.3)	<0.01	64 (5.7)	82 (3.8)	<0.01	121 (36.6)	169 (34.8)	0.60

<1.5 km.

Asymptomatic controls.

Both unadjusted and adjusted models revealed significant associations between the mode of transportation and current asthma, lifetime asthma, and symptomatic controls (Table 3). Adjusted models suggested that compared to asymptomatic controls, children with current and lifetime asthma had higher odds of being driven by car [odds ratio (OR) 1.29, 95% CI 1.01–1.65, and OR 1.26, 95% CI 1.01–1.56 respectively], whereas symptomatic controls were both more likely to be driven by either car (OR 1.34, 95% CI 1.14–1.59) or school bus (OR 1.30, 95% CI 0.99–1.71). When the analysis was stratified by the living distance from school, statistically significant associations were found between symptomatic controls living within walking distance of their school and being driven by car (OR 1.30, 95% CI 1.09–1.56) or school bus (OR 1.48, 95% CI 1.01–2.17). Associations between current or lifetime asthma and being driven by car lost statistical significance. Instead, adjusted models revealed stronger associations between current or lifetime asthma and being driven only by school bus (OR 1.71, 95% CI 1.00–2.92 and OR 1.71, 95% CI 1.06–2.76, respectively) (Table 4). No statistically significant associations were found among children living further than walking distance of their school. Adjusted models included age, sex, history of parental asthma, child being born in Canada, and income adequacy.

Table 3.

Unadjusted and Adjusted Associations Between Mode of Transportation to School and Asthma Status in Toronto School Children Aged 5–9 in 2006

	OR (95% confidence limits)
Mode of transportation to school	Unadjusted	Adjusted ^a
Current asthma versus asymptomatic controls (n=3,354)
Walk to school	Ref	Ref
Driven to school in car	1.28 (1.05–1.57)	1.29 (1.01–1.65)
Driven to school in school bus	1.36 (1.00–1.85)	1.27 (0.85–1.91)
Lifetime asthma versus asymptomatic controls (n=3,567)
Walk to school	Ref	Ref
Driven to school in car	1.21 (1.01–1.44)	1.26 (1.01–1.56)
Driven to school in school bus	1.23 (0.93–1.62)	1.22 (0.85–1.75)
Symptomatic controls versus asymptomatic controls (n=4,293)
Walk to school	Ref	Ref
Driven to school in car	1.30 (1.13–1.49)	1.34 (1.14–1.59)
Driven to school in school bus	1.47 (1.19–1.82)	1.30 (0.99–1.71)

Adjusted for age, sex, history of parental asthma, child being born in Canada, and income adequacy, and living within walking distance of school.

OR, odds ratio.

Table 4.

Adjusted Associations Between Mode of Transportation to School and Asthma Status in School Children Aged 5–9 Who Participated in the 2006 T-CHEQ Study Stratified by Living Distance from School

OR (95% confidence limits)
	Home within walking distance of school^a (n=2,715)		Home further than walking distance^b of school (n=635)
Mode of transportation to school	Unadjusted	Adjusted	Unadjusted	Adjusted
Current asthma versus asymptomatic controls (n=3,354)
Walk	Ref	Ref	Ref	Ref
Driven by car	1.20 (0.95–1.50)	1.23 (0.94–1.59)	1.34 (0.65–2.76)	1.41 (0.57–3.48)
Driven by school bus	1.72 (1.09–2.72)	1.71 (1.00–2.92)	1.04 (0.48–2.25)	1.03 (0.40–2.65)
Lifetime asthma versus asymptomatic controls (n=3,567)
Walk	Ref	Ref	Ref	Ref
Driven by car	1.18 (0.97–1.44)	1.23 (0.98–1.55)	1.01 (0.56–1.84)	1.00 (0.48–2.06)
Driven by school bus	1.61 (1.07–2.44)	1.71 (1.06–2.76)	0.77 (0.40–1.46)	0.73 (0.34–1.56)
Symptomatic controls versus asymptomatic controls (n=4,293)
Walk	Ref	Ref	Ref	Ref
Driven by car	1.23 (1.05–1.44)	1.30 (1.09–1.56)	1.35 (0.81–2.27)	1.62 (0.87–3.01)
Driven by school bus	1.66 (1.18–2.33)	1.48 (1.01–2.17)	1.40 (0.82–2.40)	1.35 (0.71–2.54)

Adjusted for age, sex, history of parental asthma, child being born in Canada, and income adequacy.

According to Toronto school board policies, children who live within 1.5 km of their school are within reasonable walking distance and do not qualify for board-provided transportation with the exception of health and safety concerns.^20,21

T-CHEQ, Toronto Child Health Evaluation Questionnaire.

The mode of transportation was re-examined in 194 children who completed the 2010 follow-up study (93 with asthma and 101 controls). Compared to data from 2006, no statistically significant differences in the mode of transportation were found in the total 2010 subsample or when the data were stratified by the proportion of children walking with or without current or lifetime asthma (Table 5). Put in another way, these children were no more likely to walk to school as they got older.

Table 5.

Mode of Transportation Frequencies in 2006 and at Follow-Up in 2010 (n=193)

	Year
Mode of transportation to school	2006	2010	P value^a
Total population (n=193)
Walk to school	92 (47.7)	95 (49.2)	0.59
Driven to school in car	74 (38.3)	69 (35.8)	0.43
Driven to school in school bus	27 (14.0)	29 (15.0)	0.68
Current asthma (n=69)
Walk to school	30 (43.5)	31 (44.9)	0.74
Driven to school in car	29 (42.0)	26 (37.7)	0.41
Driven to school in school bus	10 (14.5)	12 (17.4)	0.41
Lifetime asthma (n=93)
Walk to school	45 (48.4)	46 (49.5)	0.76
Driven to school in car	37 (39.8)	33 (35.5)	0.32
Driven to school in school bus	11 (11.8)	14 (15.1)	0.26
Symptomatic controls (n=69)
Walk to school	30 (43.5)	31 (44.9)	0.74
Driven to school in car	29 (42.0)	26 (37.7)	0.41
Driven to school in school bus	10 (14.5)	12 (17.4)	0.41
Asymptomatic controls (n=48)
Walk to school	27 (56.3)	28 (58.3)	0.76
Driven to school in car	15 (31.3)	16 (33.3)	0.76
Driven to school in school bus	6 (12.5)	4 (8.3)	0.41

By McNemar's test for matched pairs.

Discussion

Our study shows that the proportion of children who participate in active travel to school in Toronto is higher than that reported in U.S. studies (4.2%–23%),^12,27,28 but similar to those in European studies (54.5%–69%).^29–31 Consistent with the literature, the living distance from school was shown to be an important covariate in the association between asthma and the mode of transportation to school.¹² Novel findings were that Toronto school children with asthma or asthma symptoms had higher odds of being driven to school by either car or school bus, regardless of the living distance from school. Although small in magnitude, this association may become increasingly important as the trend of active transportation to school continues to decline.

Several possible mechanisms exist to explain this association. Exercise induced symptoms due to inadequately controlled asthma, as well as parental beliefs that exercise may be dangerous for their child or can worsen asthma,¹² may incline parents to have their children driven to school. Parental concerns about their child's maturity and cognitive ability regarding navigating their way to school safely have been shown to influence the mode of transportation.³² There is no evidence to suggest that this concern is more prevalent among parents of children with asthma. However, because the children in our study were young, it stands to reason that their parents may not be keen on them walking to school. Our nested case–control follow-up analysis conducted in 2010 (Table 3) showed that children were no more likely to walk to school when they were aged 9 to 12, which is consistent with other studies that have shown children are even less likely to walk to school as they age.^13,33

The possibility that the mode of transportation may be affecting asthma status cannot be ruled out. Asthma status may be affected by decreased physical activity, or increased exposure to traffic-related air pollutants. Our study found no association between ambient TRAP exposure and the mode of transportation to school.¹² For both car and school bus commutes, previous studies have highlighted the significance of the traffic type and speed in determining pollutant concentrations inside and outside vehicles.^34,35 Although we were not able to measure the air pollution levels specific to the modes of transportation of our sample, student Transportation Services of the Toronto school boards have recently implemented guidelines²⁵ to ensure that older school buses are phased out, since the age of the vehicle influences in-cabin pollutant concentrations,³⁴ and the in-cabin air quality of new school buses may be better than the outdoor air quality while the windows are closed.³⁴

Children with asthma and symptomatic controls who live within 1.5 km of their school were more likely to be driven. Being driven by the school bus even while living within the boundary making them ineligible for the service, suggests exceptions to bussing policies were made for these children, particularly those with a physician's diagnosis of asthma. Neighborhood walkability has not been associated with asthma and is ultimately less important to the final mode choice than are busy work schedules and extracurricular activities, which often influence parental decisions.³² Parents of children with asthma may be more likely to petition the school board to have their child driven by school bus simply as a matter of convenience.^32,36

The strength of this study is its urban population-based sampling methodology. The use of ISAAC methodologies to determine the asthma status also ensures comparability of our results to the international literature. This study also collected the exact home and school address information, which allowed for an accurate and systematic way of measuring the distance from school for each child. However, without detailed information regarding the child's route to school, we were unable to consider the travel time in our models, which may be an important explanatory variable to consider. We also cannot determine the direction of the association between asthma and the mode of transportation, or be sure that it is not being mediated by a third variable such as habitual physical activity because of the cross-sectional nature of the data. The height and weight data were collected by a parental report, which may not be as reliable as measured parameters.^37–39 Canadian data have shown that the use of this data among 6- to 8-year-old children may bias the results.⁴⁰ Since the associations between the body mass index (BMI) and both active transport and asthma are not consistent,^41–43 it is unlikely that the BMI is a confounder that would be responsible for the main results observed in our study.

Conclusion

The benefits of regular physical activity among children with asthma are well known, but results of this study indicate that children with asthma or asthma symptoms are more likely to be driven to school and less likely to walk than their asymptomatic peers, regardless of the living distance from school. Prospective studies should be conducted to further elucidate the etiological mechanisms and direction of the association between the mode of transportation to school and the development of childhood asthma.

Authors' Contributions

Richard G. Foty participated in the study design, data collection, analysis, and the writing and review of the manuscript, Kathleen Nelligan participated in analysis, writing and review of the manuscript, doctors David M. Stieb and Teresa To participated in the interpretation of the results and review of the final manuscript and Dr. Sharon D. Dell who designed the study, participated in the interpretation of the results and oversaw the review of the final manuscript.

Footnotes

Acknowledgment

Funding for this study was generously provided by Health Canada.

Author Disclosure Statement

All authors have no potential conflicts of interest to report.

References

Becker

, Berube

, Chad

, Dolovich

, Ducharme

, D'Urzo

et al. Canadian Pediatric Asthma Consensus Guidelines, 2003 (updated to December 2004): introduction. CMAJ, 2005; 173,6 Suppl:S12–S14.

Lougheed

, Lemière

, Dell

, Ducharme

, FitzGerald

, Leigh

et al. Canadian Thoracic Society Asthma Management Continuum-2010 Consensus Summary for children six years of age and over, and adults. Can Respir J, 2010; 17:15–24.

Bateman

, Hurd

, Barnes

, Bousquet

, Drazen

, FitzGerald

et al. Global strategy for asthma management and prevention: GINA executive summary. Eur Respir J, 2008; 31:143–178.

National Heart Lung and Blood Institute. Expert Panel Report 3 (EPR-3): Guidelines for the Diagnosis and Management of Asthma-Summary Report 2007. J Allergy Clin Immunol, 2007; 120,5 Suppl:S94–S138.

Carnethon

, Gidding

, Nehgme

, Sidney

, Jacobs

Jr. , Liu

. Cardiorespiratory fitness in young adulthood and the development of cardiovascular disease risk factors. JAMA, 2003; 290:3092–3100.

Welsh

, Kemp

, Roberts

. Effects of physical conditioning on children and adolescents with asthma. Sports Med, 2005; 35:127–141.

Rasmussen

, Lambrechtsen

, Siersted

, Hansen

. Low physical fitness in childhood is associated with the development of asthma in young adulthood: the Odense schoolchild study. Eur Res J, 2000; 16:866–870.

Shaaban

, Leynaert

, Soussan

, Anto

, Chinn

, de Marco

et al. Physical activity and bronchial hyperresponsiveness. Eur Commun Respir Health Surv II Thorax, 2007; 62:403–410.

Mendes

, Almeida

, Cukier

, Stelmach

, Jacob-Filho

, Martins

et al. Effects of aerobic training on airway inflammation in asthmatic patients. Med Sci Sports Exerc, 2011; 43:197–203.

10.

Dogra

, Kuk

, Baker

, Jamnik

. Exercise is associated with improved asthma control in adults. Eur Respir J, 2011; 37:318–323.

11.

McDonald

. Active transportation to school: trends among U.S. schoolchildren, 1969–2001. Am J Prev Med, 2007; 32:509–516.

12.

Oreskovic

, Sawicki

, Kinane

, Winickoff

, Perrin

. Travel patterns to school among children with asthma. Clin Pediatr (Phila), 2009; 48:632–640.

13.

Buliung

, Mitra

, Faulkner

. Active school transportation in the Greater Toronto Area, Canada: an exploration of trends in space and time (1986–2006) Prev Med, 2009; 48:507–512.

14.

Ham

, Martin

, Kohl

3rd . Changes in the percentage of students who walk or bike to school-United States, 1969 and 2001. J Phys Act Health, 2008; 5:205–215.

15.

Brophy

, Cooksey

, Lyons

, Thomas

, Rodgers

, Gravenor

. Parental factors associated with walking to school and participation in organised activities at age 5: analysis of the Millennium Cohort Study. BMC Public Health, 2011; 11:14.

16.

Ahlport

, Linnan

, Vaughn

, Evenson

, Ward

. Barriers to and facilitators of walking and bicycling to school: formative results from the non-motorized travel study. Health Educ Behav, 2008; 35:221–244.

17.

Statistics Canada. Selected trend data for Toronto (City), 2006, 2001 and 1996 censuses. 2006[cited 2008 Sept 17]; Available from:www12.statcan.ca/english/census06/data/trends/Table_1.cfm?TID=0&T=CSD&PRCODE=35&GEOCODE=20005&geosubCSD=Submit&GEOLVL=CSD

18.

Dell

, Foty

, Gilbert

, Jerret

, To

, Walter

et al. Asthma and allergic disease prevalence in a diverse sample of Toronto school children: results from the Toronto Child Health Evaluation Questionnaire (T-CHEQ) Study. Can Respir J, 2010; 17:e1–e6.

19.

Asher

, Keil

, Anderson

, Beasley

, Crane

, Martinez

et al. International Study of Asthma and Allergies in Childhood (ISAAC): rationale and methods. Eur Respir J, 1995; 8:483–491.

20.

Pekkanen

, Sunyer

, Chinn

. Nondifferential disease misclassification may bias incidence risk ratios away from the null. J Clin Epidemiol, 2006; 59:281–289.

21.

Yang

, To

, Foty

, Stieb

, Dell

. Verifying a questionnaire diagnosis of asthma in children using health claims data. BMC Pulm Med, 2011; 11:52.

22.

Statistics Canada. National Population Health Survey Household Component Cycle 5 (2002–2003) Derived Variables Documentation (Specifications) Cycles 1 to 5. Ottawa, ON, 2004.

23.

Centers for Disease Control: Division of Nutrition PA, and Obesity, National Center for Chronic Disease Prevention and Health Promotion. A SAS Program for the CDC Growth Charts. 2012[cited 2012 August 15]; Available fromwww.cdc.gov/nccdphp/dnpao/growthcharts/resources/sas.htm

24.

Jerrett

, Arain

, Kanaroglou

, Beckerman

, Crouse

, Gilbert

et al. Modeling the intraurban variability of ambient traffic pollution in Toronto, Canada. J Toxicol Environ Health A, 2007; 70:200–212.

25.

Dove

. Healthy School Bus Plan. Toronto District School Board, Toronto Catholic District School Board, 2006.

26.

Toronto Catholic District School Board. TCDSB Policy Register: Transportation—S.T.01. Board

TCDS

ST012007.

27.

Centers for Disease Control and Prevention. School Transportation Modes—Georgia, 2000. MMWR Morb Mortal Wkly Rep., 2002; 51:704–705.

28.

Staunton

, Hubsmith

, Kallins

. Promoting safe walking and biking to school: the Marin County success story. Am J Public Health, 2003; 93:1431–1434.

29.

Grize

, Bringolf-Isler

, Martin

, Braun-Fahrlander

. Trend in active transportation to school among Swiss school children and its associated factors: three cross-sectional surveys 1994, 2000 and 2005. Int J Behav Nutr Phys Act, 2010; 7:28.

30.

DiGuiseppi

, Roberts

, Li

, Allen

. Determinants of car travel on daily journeys to school: cross sectional survey of primary school children. BMJ, 1998; 316:1426–1428.

31.

Cooper

, Page

, Foster

, Qahwaji

. Commuting to school: are children who walk more physically active? Am J Prev Med, 2003; 25:273–276.

32.

Faulkner

, Richichi

, Buliung

, Fusco

, Moola

. What's “quickest and easiest?”: parental decision making about school trip mode. Int J Behav Nutr Phys Activity, 2010; 7:62.

33.

Pabayo

, Gauvin

. Proportions of students who use various modes of transportation to and from school in a representative population-based sample of children and adolescents, 1999. Prev Med, 2008; 46:63–66.

34.

Zhang

, Zhu

. Measurements of ultrafine particles and other vehicular pollutants inside school buses in South Texas. Atmos Environ, 2010; 44:253–261.

35.

Knibbs

, de Dear

, Morawska

. Effect of cabin ventilation rate on ultrafine particle exposure inside automobiles. Environ Sci Technol, 2010; 44:3546–3551.

36.

Zhu

, Lee

. Correlates of walking to school and implications for public policies: survey results from parents of elementary school children in Austin, Texas. J Public Health Policy, 2009; 30,Suppl 1:S177–S202.

37.

Brettschneider

, Ellert

, Schaffrath Rosario

. Comparison of BMI derived from parent-reported height and weight with measured values: results from the German KiGGS study. Int J Environ Res Public Health, 2012; 9:632–647.

38.

Huybrechts

, Himes

, Ottevaere

, De Vriendt

, De Keyzer

, Cox

et al. Validity of parent-reported weight and height of preschool children measured at home or estimated without home measurement: a validation study. BMC Pediatr, 2011; 11:63.

39.

Garcia-Marcos

, Valverde-Molina

, Sanchez-Solis

, Soriano-Perez

, Baeza-Alcaraz

, Martinez-Torres

et al. Validity of parent-reported height and weight for defining obesity among asthmatic and nonasthmatic schoolchildren. Int Arch Allergy Immunol, 2006; 139:139–145.

40.

Shields

, Gorber

, Janssen

, Tremblay

. Obesity estimates for children based on parent-reported versus direct measures. Health Rep, 2011; 22:47–58.

41.

Gordon-Larsen

, Nelson

, Beam

. Associations among active transportation, physical activity, and weight status in young adults. Obesity, 2005; 13:868–875.

42.

Fulton

, Shisler

, Yore

, Caspersen

. Active transportation to school: findings from a national survey. Res Q Exerc Sport, 2005; 76:352–357.

43.

Heelan

, Donnelly

, Jacobsen

, Mayo

, Washburn

, Greene

. Active commuting to and from school and BMI in elementary school children-preliminary data. Child Care Health Dev, 2005; 31:341–349.