Assessment of physical education time and after-school outdoor time in elementary and middle school students in south Mexico City: the dilemma between physical fitness and the adverse health effects of outdoor pollutant exposure
ELEVATED LEVELS OF AIR POLLUTANTS are associated with adverse health effects. (1-9) There is increasing concern about the long-term effects of chronic exposure of healthy children to atmospheric pollutants and the effects on the respiratory system that result from this exposure. (10-13) Children and adolescents are particularly at risk because they engage in play and outdoor physical activities at times of the day when pollutant concentrations are at maximum levels. The results of epidemiological studies strongly suggest that children have an increased risk of morbidity from photochemical smog and particulate air pollution. (3,4,6,11)
The atmosphere in south Mexico City (SMC) is a complex mixture of pollutants, including ozone–the predominant pollutant, with concentrations that exceed 0.08 ppm for an average of 4.3 hr/day–particulate matter (PM), formaldehyde, acetaldehyde, nitrogen oxides, volatile organic compounds, nonmethane organic compounds, alkaline hydrocarbons, metals, benzene, and other pollutants that have not been completely characterized. (14-19) Elementary school children who live in SMC have previously shown acute decrements in forced expiratory volume in 1 sec (FE[V.sub.1.0]) that correlated with daily ambient ozone levels. (20) In addition, a correlation exists between reduced peak expiratory flow and exposures to ozone and PM within the previous 1 to 2 wk in SMC children. (21) SMC infant mortality in excess of 6.9% is strongly associated with an increment of 10 [micro]g/[m.sup.3] in fine PM (i.e., PM with a diameter of 2.5 [micro]m or less) on the 3-5 days that precede an infant’s demise. (22) Children may be especially at risk for the harmful effects of air pollutants because they have higher ventilation rates, the result of which is an increase in pollutant dose delivered to the distal lung. (23) Children have an increased need for oxygen relative to their size, and childhood is a crucial period for lung development. (24,25) The number of alveoli present at birth increases 10-fold during the first 5-10 yr of life. (26) Moreover, the sites of alveolar development include the distal bronchioles–the target area for ozone and PM, which are 2 of the main criteria pollutants in Mexico City that exhibit sustained concentrations above their respective United States Environmental Protection Agency (EPA) National pre-1997 Ambient Air Quality Standard (NAAQS) levels. (12,27-29)
Several factors can influence the distribution and severity of lung parenchymal lesions induced by air pollutants. One important factor is the integrity of the mucociliary epithelium. SMC children exhibit severe ultrastructural alterations of their nasal epithelium, and they have acquired prominent ciliary defects that severely compromise their mucociliary defense mechanisms. (30) An important concern is the association between poor mucociliary transport rate and prolonged PM retention in the nose, as well as the increased likelihood of recurrent chronic upper and lower respiratory infections and bronchiectasis. (31,32) Children exposed to tobacco smoke, who have little fresh fruit and vegetable intake, low antioxidant levels, high polyunsaturated fat intake, or who have inherited particular alleles for genes involved in lung responses to injury, may be at increased risk for lung pathologies. (10,33,34) We have recently shown that clinically healthy SMC children with negative histories of tobacco exposure, prematurity, atopias, asthma, or chronic respiratory diseases exhibit radiological evidence of lung hyperinflation that is associated significantly with exposure to the region’s atmosphere. (12) Lung hyperinflation among this highly exposed cohort suggests the presence of small-airway disease, and this condition could indicate an impending increase in lung diseases. Little is known about the long-term health implications of living in areas where ambient ozone concentrations and other pollutants exceed current standards for ambient air over a period of many years. (10,35,36)
It is clear, however, that childhood and adolescent physical activities are associated with a range of beneficial health and fitness outcomes. (37) Physical inactivity is a major determinant of obesity, chronic disease, and disability. A crucial issue in child and adolescent populations in Mexico City is how a balance can be achieved between the physical activity needed to avoid adverse health consequences of inactivity, and the potential adverse, long-term health effects of exposure to air pollutants. In the present study, we attempted to determine the following: (1) the time(s) of day that students in public elementary and middle schools in SMC had physical education classes, (2) the number of hours and the time(s) of day students spent outdoors when they were not attending school, (3) the number of hours they spent viewing television, (4) personal exposure to environmental tobacco smoke (ETS), and (5) body mass index ([BMI] = weight/height squared). In this study, we ultimately sought to use our results as a basis for suggesting prevention strategies to protect the 6 million children currently living in metropolitan Mexico City.
Materials and Method
Sample. Appropriate institutional and passive parental consent were obtained for students who participated in the data collection. Data were collected in SMC from 4th through 9th graders who attended the morning shift in public elementary and middle schools. We considered 3 factors during the selection process of children from grades 4-9. (1) Age range: the assessment of children and adolescents is challenging, owing to the developmental limits on children’s cognitive abilities to estimate time and recall events. Children’s cognitive capacities are age-dependent; therefore, accurate recall of times and activities is most likely better for children 9 yr of age and older than for younger children. (2) At-risk group: previous studies of SMC children have shown that respiratory symptomatology is seen predominantly in children 7-13 yr of age, and it is associated significantly with outdoor exposures. (12) (3) Developmental stage: we explored the differences in parameters evaluated in elementary vs. middle school students–at a stage of important physiological, educational, and social changes. The type of school sampled may be an indicator of a student’s socioeconomic status (i.e., low and middle-low), and in our study, it was representative of 78% of the elementary and middle school population in Mexico City.
In SMC, 17 schools with morning shifts were selected in accordance with a stratified sampling procedure. We used a 2-step cluster sampling to select classes from grades 4-9. From an original population of 12,563 students, 1,382 students were selected randomly. We invited the students from the selected classrooms to participate in the study. Survey procedures protected each student’s privacy, and participation was “anonymous.” A self-administered questionnaire was presented to the class by the senior investigators and was proctored in the classroom by trained data collectors. Students required 5-8 min to complete the questionnaire. The regular classroom teachers and physical education teachers did not assist in administering the questionnaire, thus avoiding biased responses. Following administration of the questionnaire, anthropometric data (i.e., height and weight) for participating students were recorded by a dental student. Shoes were removed prior to the taking of measurements, all students wore a similar 2-piece school uniform, and all measurements were completed prior to snack/recess time. We measured the height of students by taping a new plastic tape measure against a wall and placing a rigid right-angle against the tape measure. The angle was moved down the tape until its horizontal surface touched the child’s head. Hairstyles were adjusted so as not to interfere with the measurements. Weights were measured with a Healthometer 402 KLS scale (Snoqualmie, Washington). If requested, subjects were told their height and weight.
Data collection and definitions. A total of 1,159 students completed a self-administered, 20-item questionnaire, and their anthropometric measurements were taken. Five of the survey items determined school grade, gender, age, duration of time he or she had lived in the city, and parents’ occupations. We used the remaining 15 items to measure (1) time spent outdoors (i.e., determination of physical activities in which he or she was engaged), (2) behaviors and characteristics that revealed leisure-time activities (e.g., television viewing), (3) exposure to ETS, and (4) personal smoking habits. Students were asked how many hours each week they attended physical education classes and the time of day these classes occurred. We also used responses to survey items to determine levels of physical activity (i.e., light, moderate, or heavy) that were associated with various physical education classes. Students were asked to describe their method of transport to school, type of transport, time spent outdoors–after school during the school week and on weekends–and, if they were involved in exercise(s) away from the school grounds and the timing of such exercise(s). Students were asked how many hours each day they watched television and/or played video games, whether they lived in a home that housed smoker(s), and whether he or she smoked. Finally, the student was asked “if a doctor has told you that you have–or have had–asthma?” We included this question because we wanted to determine the percentage of students who were aware of such a diagnosis in this population. Researchers provided examples, answered questions, briefly described the symptomatology commonly associated with asthma, and worked in the classroom with individual students, when necessary.
For the BMI definitions, we followed Gomez-Peresmitre’s validated BMI for Mexican preadolescents, which is a combination of Vargas and Casillas (38) and Gomez-Peresmitre values. (39) Low weight was defined as a BMI 27. Sedentary activity involved no weight transfer; minimal activity involved nonstrenuous arm, leg, and trunk movements (e.g., stretching). Light to moderate physical activity was defined as activity requiring sustained, rhythmic muscular movements performed for at least 30 min per occasion, 5 or more times per week. Vigorous physical activity was defined as rhythmic, repetitive physical activity for which large muscle groups were used at 60% or more of the maximum heart rate for age. A list of common activities for Mexican children was provided as examples of the different activity levels. The list was adapted from the Weston et al. (40) instrument for measurement of physical activity in youth; the instrument was provided by Russell R. Pate, University of South Carolina at Columbia, South Carolina. The elementary and middle school principals and physical education teachers reported that physical education was scheduled twice a week, 50 min per class. In elementary school, recess was scheduled 5 times weekly for 25-30 min, usually between 10:30 A.M. and 11:00 A.M. All schools surveyed employed certified physical education teachers.
Statistical analysis. Statistical analysis was performed with the Instat program (Graph Pad [San Diego, California]) and the SAS Program version 8.1 (SAS [Cary, North Carolina]). The statistical association between continuous variables (e.g., age, gender, time outdoors) was evaluated with Spearman’s correlations for all participants as a group, and also for each of the 4 combined subgroups of gender and level of education. We adjusted a multiple-regression model for the effect of total time spent outdoors each week (i.e., the sum of “outdoor time” during the weekdays and on weekends) on the variables of age, gender, BMI, television time, elementary vs. middle school, and ETS exposure (i.e., the participant admits she/he smokes and/or is exposed to cigarette smoke in the home). Fisher’s exact test was used for the differences in the numbers of overweight and obese children in elementary vs. middle school. Data are presented as means [+ or -] standard deviations (SDs), and a value of p < .05 was used to determine statistical significance.
Study area. Mexico City extends beyond 2,000 [km.sup.2] and is located in a high mountain basin, 2,250 m above sea level. The city has a mild tropical climate all year long, with a mean annual temperature of 16 [degrees]C and mean rainfall of 860 mm. Sunshine, temperature inversions, light winds, a basin setting, overcrowded population, heavy traffic, urban leakage of liquefied petroleum gas, and intense industrial activity contribute to Mexico City’s unique environment in which complex photochemical reactions produce oxidant chemicals and PM. Ozone concentrations in the city exceeded the U.S. NAAQS on 71% of days in 1986 and 98% of days in 1992, with peak values as high as 0.48 ppm. (16) Ozone is produced when sunlight triggers chemical reactions involving reactive hydrocarbons and nitrogen oxides. As a result of wind transport of the mass pollutants emitted in the industrial north and central parts of Mexico City, the maximal concentrations of ozone precursors appear downwind of the emission zones, toward the southern part of the urban area: southwest and southeast Mexico City. (14) In winter and early spring, the atmosphere is characterized by frequent temperature inversions and stagnant conditions. (17) Residents in SMC are exposed to photochemical smog, (14) airborne particles, (15) formaldehyde and acetaldehyde, (17) and alkane hydrocarbons (e.g., propane, isobutane, n-butane). (16)
To meet the NAAQS standard for ozone, the 3-yr average of the 4th-highest daily maximum 8-hr average ozone concentrations measured at each monitoring station within an area must not exceed 0.08 ppm. Daily ozone concentrations above the current NAAQS are recorded in SMC throughout the year, and P[M.sub.2.5] and P[M.sub.10] routinely exceed their respective NAAQS annual arithmetic means (of P[M.sub.10] = 50 [micro]g/[m.sup.3] and P[M.sub.2.5] = 15 [micro]g/[m.sup.3]) at P[M.sub.10] = 78 [micro]g/[m.sup.3] and P[M.sub.2.5] = 21.6 [micro]g/[m.sup.3]. (15,19) Formaldehyde and acetaldehyde concentrations in SMC are reportedly in the range of 5.9-110 ppb and 2-66.7 ppb, respectively. Maximal peaks are recorded between 8:00 A.M. and 10:00 A.M. for acetaldehyde and between 10:00 A.M. and 12:00 P.M. for formaldehyde. (17) Formaldehyde concentrations are higher on sunny days, coinciding with atmospheric stability and heavy smog conditions. These SMC outdoor formaldehyde and acetaldehyde values are considered among the highest reported in urban air worldwide. (17) The ozone and P[M.sub.10] concentrations from 3 representative school days in February, June, and October in SMC are shown in Figure 1. The SMC average 24-hr values of ozone and P[M.sub.10] are illustrated in Figure 2 for the years 1999 and 2000.
[FIGURES 1-2 OMITTED]
The student sample included children who lived in 4 SMC geopolitical counties (i.e., delegaciones): Coyoacan, Tlalpan, Xochimilco, and Magdalena Contreras. The demographic characteristics of these counties are shown in Table 1. According to the 1995 Census, 302,591 children between the ages of 6 yr and 14 yr attended elementary and middle schools in the 4 counties identified above (data taken from SEDECO Agenda, 1998). The population projections for 2000, 2005, and 2010 for Mexico City (Federal District) are shown in Table 2. For the Federal District alone–excluding metropolitan Mexico City–the population projections indicate that SMC will be inhabited by [greater than or equal to] 2 million children 0-14 yr of age during the next decade.
Study population. A total of 1,159 students–559 elementary and 600 middle school students–were included in the study. The main study results are shown in Table 3. A significant difference was seen in the number of hours spent outdoors, both during weekdays and on weekends, between elementary and middle school students (p = .002 for school days and p < .0001 for weekends). When results were examined by gender, boys spent more time outdoors on weekdays and on weekends (p < .0001), regardless of age, than did girls. The outdoor time periods extended from the time the students finished school (12:30 P.M.-1:00 P.M. for elementary school students and 1:30 P.M.-2:30 P.M. for middle school students) to 6:00 P.M. The outdoor exposure times coincided with concentrations of ambient air pollutants that exceeded their respective NAAQS (Fig. 2). We also found a significant difference between elementary and middle school students with respect to the time taken to travel from home to school, and the mode of transportation also differed significantly. Travel time was longer for middle school students because there were fewer middle schools than elementary schools in each county; 76% of elementary school children walked to school vs. 34% in middle school. Public transportation was used most often by elementary and middle school students; however, no school buses are available to public school children in Mexico. On the other hand, there was no difference between elementary and middle school students in the number of hours spent viewing television, nor did we see any significant differences with respect to gender. General play, chasing games, soccer, jump rope, brisk walking, and running were the most common activities during recess for elementary school children. Physical education classes for both groups involved light and moderate physical activities, including calisthenics, jogging/walking, free play, soccer, jump rope, and volleyball. Thirty-two percent of elementary and 61% of middle school students had physical education classes after 11:00 A.M. (Figs. 3 A and B).
Low-weight children were seen most often among the elementary school populations. We found a significant increase in the number of middle school students who were overweight and obese (p = .01 [Table 4]). Tobacco exposure at home was recorded in 49.89% of households (48.00% in elementary student households and 51.78% in middle school student households); 7.3% of middle school students reported that they smoked vs. only 1 child in elementary school who reported this behavior. We made no attempt to distinguish between experimenters and current tobacco users. We found no correlations between BMI and time spent viewing television, time spent outdoors (both weekdays and weekends), or exposure to ETS. Awareness that a physician had given an asthma diagnosis was low in both groups: 0.89% vs. 3.90% for elementary and middle school students, respectively.
The health effects of chronic exposure to significant concentrations of air pollutants on Mexico City’s millions of children should be of clinical, public health, and regulatory concern. Children who live in SMC are exposed daily to J complex mixture of pollutants, including high ozone concentrations, P[M.sub.10], formaldehyde, and acetaldehyde, plus a myriad of other pollutants that are not monitored. The results of this study show that public elementary and middle school students in SMC spend significant amounts of time outdoors–both during and after-school hours–engaged in a variety of mostly light-to-moderate physical activities at times of day when air pollutants are above their respective NAAQS. These children are exposed to significant concentrations of ozone and P[M.sub.10] during physical education classes, and their exposure continues after school hours. Our observation that boys spent significantly more time outdoors than girls is important in view of the finding by Fajardo-Gutierrez et al., (41) who determined that the highest incidence of malignant neoplasms (i.e., leukemias, lymphomas, and central nervous system tumors) occurs in boys living in SMC. Public schools in SMC do not have indoor exercise facilities, and very few children from public schools exercise indoors after school (e.g., swimming, gymnastics, martial arts) because economic constraints exist. Results of recent studies suggest that residence location, time spent outdoors, and types of outdoor activities are the major determinants of “effective” lifetime ozone exposure. (35) Moreover, data from available monitoring sites appear adequate for the estimation of spatial patterns of long-term average ambient ozone and PM concentrations. (13,42-45) During exposure to ozone, PM, and other air pollutants, health risk depends on several factors, including inhaled dose, which in turn depends on concentration, time, and ventilation rates. (46) Studies in which ozone exposure has been assessed in California children (13) indicate that indoor ozone concentrations are dependent on outdoor values and, importantly, on how homes are ventilated. In California homes ventilated by open windows (the norm in Mexico City), higher ozone values were recorded indoors during the ozone season. These indoor values corresponded to approximately 30-40% of the outdoor concentrations. (13) This finding will translate in Mexico City homes and schools to indoor values above 0.08 ppm (the current NAAQS for ozone) when outdoor values are at or above 0.2 ppm. Gold et al., (47) who studied SMC school classrooms, determined that significant concentrations of indoor ozone are attained if the windows and doors of a classroom remain open during outdoor peak pollution hours.
Two unsurprising findings in our study were that (1) 50% of students were exposed to ETS at home, and (2) 7.3% of middle school students smoke. Smoking rates have increased significantly in Mexican children and adolescents; from 1988 to 1993, the prevalence of smoking among minors aged 12-17 yr increased from 6.6% to 9.6%.48 Smoking prevalence in adult men is 40%, and in adult women the prevalence is 1 7.6%. (49) Aggressive marketing and promotional activities by transnational tobacco companies have contributed to these adverse trends. (50) Illegal sales of cigarettes to minors is a common phenomenon in Mexico City. (48) Epidemiological data suggest that exposure to ETS can increase the risk of lung cancer, cardiovascular and chronic lung diseases, intrauterine growth retardation, and sudden infant death syndrome. (34) In addition, children have increased rates of lower respiratory illness, middle ear effusions, and asthma. (33)
Malnutrition remains a severe national problem in the pediatric Mexican population (51); 38% of the children in our study were classified as being low-weight in accordance with reference BMI values, whereas 22% were either overweight or obese. Food insufficiency and/or poor nutritional habits affected 60% of the children in our study. In a recent study by Hernandez et al., (52) 24% of 461 Mexico City children (i.e., 9-16 yr of age) were classified as obese on the basis of height, weight, and triceps skinfold measurements. Currently, obesity among Mexican women is highly prevalent, especially in individuals from urban communities. In younger Mexican adults, obesity is associated with hypertension, higher insulin levels, and central adiposity. (53)
The results of the apparently odd question in this study (i.e., “if a doctor has told you that you have or had asthma”) were low, as expected on the basis of our average pediatric practice in SMC (personal communication by Dr. Gildardo Valencia). Of the 1,159 children studied, only 55 answered affirmatively–a rather low number compared with the prevalence of physician-diagnosed asthma in school children in developed countries. (54) In this regard, current theories that implicate absence of childhood infections as a factor in the development of asthma (55)–and exposure of young children to older siblings against the development of asthma (56)–are, in our view, relevant to our populations.
In this study, we sought to formulate suggestions, derived from the results of our research, that would ultimately protect our populations. In keeping with this goal, we recommend the following.
Air pollution forecasting. This activity would involve the creation of a reliable ozone and PM forecasting center, where a color-coded air quality index guide would be readily available daily to the general population and to the school system. Forecasts should indicate air quality levels, unfavorable weather conditions, potential health effects, and recommended actions for the current date for each geographical sector of the city. The Air Quality Index should be based on the concentrations of ozone (in ppm) and PM (in [micro]g/[m.sup.3]). People should be made aware that vigorous outdoor exercise–which increases the dose of pollutants delivered to the respiratory tract–should be avoided at times during which there are increased pollutant concentrations. (57)
Quality physical education programs. We should offer excellent physical education programs–defined as physical education and physical activities that help to develop lifetime habits and behavior patterns that contribute to physical well-being. (58) The Board of Education should offer a minimum of 3 physical education classes each week for all students, to promote lifelong fitness and cardiovascular health. The classes should be scheduled prior to the peak hours for the main criteria pollutants, in accordance with each geographical sector.
Nutritional education. We must target nutritional education and emphasize balanced diets, beginning in elementary school. Mexico City residents have access to affordable fresh fruits and vegetables throughout the year; we must educate families to eat healthy food.
Indoor facilities. Indoor facilities should be available for activities and physical safety in the critical after-school hours. This approach is particularly important for boys because they spend more time outdoors and are likely to be engaged in competitive sports. Furthermore, evidence currently exists that boys who reside in SMC have a high incidence of malignant neoplasms (e.g., leukemias, lymphomas, central nervous system tumors (41)–an epidemiological observation that could be related to exposure to potentially carcinogenic substances in polluted air.
Pollution control. Researchers should advise the public to replace broken windows, to keep doors and windows closed during peak pollutant hours, and to open windows early in the morning or at night. The same practices should apply to schools. (47) These simple measures will reduce indoor ozone levels considerably.
Smoke-free schools. The Board of Education should firmly establish the policy that all school buildings and grounds are to be smoke-free. This policy should reinforce the teaching of the harmful effects of smoking, and it should provide a healthful teaching and learning environment. The policy should apply to all personnel and all visitors to school property. A separate policy should prohibit smoking by students on school grounds.
Healthy food choices at school. In most elementary schools visited by the researchers, mothers were in charge of a cooperative store where homemade food and fresh fruits were available to the children during recess. This practice should be encouraged, and emphasis should be placed on making seasonal fresh fruits and vegetables available to the children. Teachers and administrators should discourage consumption of “junk food.”
Prevention. Ultimately, all efforts should be directed toward prevention of air-pollution-related health problems, and parents should be urged to take responsibility for their children. We must create an environment conducive to children attaining optimal health, including providing dietary advice. Both undernutrition and obesity should be addressed because of their proven relationship to morbidity and mortality. Prevention also requires that the health-care budget be given top priority. The media can aid in this effort by supporting effective television/radio/newspaper advertisements. The medical community can have a tremendous impact on the promotion of fitness and exercise, balanced diets, and tobacco abstention, both for children and for our adult population (especially the 50% of parents who expose their children to ETS).
In summary, we must educate our children and adolescents–through the school and health-care systems-to obtain necessary exercise while protecting themselves from high pollutant concentrations. We must instruct and encourage young people to be involved in lifetime fitness activities and to consume balanced diets–if our aim is to control health-care costs, reduce disease incidence, and improve the overall quality of life of our population. (59) The implications are serious when one acknowledges that diseases (e.g., coronary heart disease, hypertension, type II diabetes mellitus, osteoporosis, obesity) all have their roots in childhood behaviors.
The importance of addressing the effects on children’s health of chronic exposure to particulate and photochemical air pollution is crucial for 3 reasons: (1) exposure is ubiquitous; (2) exposure is involuntary; and (3) lung growth continues during childhood and adolescence, and there is potential for increased lung damage and risk of chronic respiratory diseases. (10) The results of animal studies have provided a clear warning: long-term exposure to pollutants, such as ozone, can have lasting and irreversible effects. (57) Furthermore, the risk effects of chronic exposure to pollution are cumulative, and loss-of-life has been estimated at 1-3 yr for lifelong residents of highly polluted U.S. cities–cities with pollution levels well below those of Mexico City. (60) The high altitude and basin topology that trap pollution, our children’s nutritional problems, the aggressive marketing of tobacco and junk-food products aimed at our young population by multinational corporations, and the lack of awareness by the population at large of the health effects of pollution all provide a critical prospect for the future of our children.
Outdoor air is a community resource–our laws establish “the right of every person to live in an optimal environment for their development, health and well-being.” (61) Regulatory agencies charged with protecting public health have a very important challenge ahead.
Table 1.–South Mexico City Child Population: 1995 *
County Population Children (6-14 yr)
Coyoacan 653,489 113,319
Tlalpan 552,516 95,212
Xochimilco 332,314 55,660
Magdalena Contreras 211,898 38,400
* Data from INEGI Conteo de Poblacion y Vivienda 1995:
Table 2.–Mexico City Population Projections *
Age groups (yr) 2000 2005 2010
0 128,797 150,581 135,043
1-5 618,956 765,423 671,633
6-11 755,261 917,015 854,316
12-14 432,710 476,907 473,379
Total 1,935,724 2,309,926 2,134,371
* CONAPO Situacion Demografica del Distrito Federal 1996:
Table 3.–South Mexico City Students in Public Elementary
and Middle Schools in Study Counties
Grade Age (yr) Weight (kg) Height (m) BMI
4-6 (n = 559)
[bar]x 10.8 41.9 1.44 19.8
SD 0.96 10.7 0.08 3.8
Males (n = 293)
Females (n = 266)
7-9 (n = 600)
[bar]x 13.6 52.9 1.57 21.1
SD 1.12 11.07 0.1 3.6
Males (n = 292)
Females (n = 308)
Outdoor time (hr)
school Week- TV time
Grade (min) days Weekends Total (hr/day)
4-6 (n = 559)
[bar]x 13.26 2.51 5.3 18.0 3.28
SD 11.9 * 1.9 3.1 10.8 1.85
Males (n = 293)
[bar]x 2.8 5.6 20.2
SD 2.2 3.2 11.56
Females (n = 266)
[bar]x 2.1 4.9 15.9
SD 1.7 2.9 9.6
7-9 (n = 600)
[bar]x 17.64 2.8 6.2 20.0 3.37
SD 13.0 1.9 3.07 11.0 1.7
Males (n = 292)
[bar]x 3.1 6.4 22.0
SD 1.9 3.04 11.3
Females (n = 308)
[bar]x 2.5 6.0 19.0
SD 1.9 3.1 10.9
Notes: [bar]x = mean, SD = standard deviation, and BMI = body
* 76% of the children walked to and from school.
([dagger]) 34% of the children walked to and from school.
Table 4.–Children’s Classification, * in Accordance with
Body Mass Index (BMI)
(n = (n =
classification ([dagger]) % No. % No.
Low < 18.9 47 262 30 180
Normal 19-22.9 34 190 45 270
Overweight 23-27 14 78 17 102
Obese > 27 5 27 8 48
* A combination of Vargas Casillas and Gomez-Peresmitre
([dagger]) Weight (in kg) / height (in [m.sup.2]).
Figure 3. Physical education class schedule for (A) elementary and (B)
middle school students. Note that 32% of elementary and 61% of middle
school children participated in physical education classes after
8-9 A.M. 28%
9-10 A.M. 21%
10-11 A.M. 19%
11 A.M.-Noon 10%
Noon-1 P.M. 22%
(B) Middle School
7-8 A.M. 15%
8-9 A.M. 13%
9-10 A.M. 4%
10-11 A.M. 7%
11 A.M.-Noon 18%
Noon-1 P.M. 19%
1-2 P.M. 24%
Note: Table made from pie chart.
This study could not have been conducted without the enthusiastic participation of young people. In addition, we thank the principals, physical education teachers, and social workers at the participating schools for their help with this study.
Submitted for publication January 17, 2001; revised; accepted for publication June 12, 2001.
Requests for reprints should be sent to Gildardo Valencia-Salazar, M.D., Ave. Gabriel Mancera #1516-1, Mexico DF, Mexico.
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Facultad de Medicina
Universidad Nacional Autonoma de Mexico
Mexico City, Mexico
Facultad de Odontologia
Universidad Nacional Autonoma de Mexico
Mexico City, Mexico
Estudios Superiores de Monterrey Campus
Ciudad de Mexico, Mexico
Escuela Secundaria Tecnica #11
Mexico City, Mexico
CARLOS F. HENRIQUEZ-ROLDAN
Facultad de Ciencias
Depto. de Estadistica
Universidad de Valparaiso
Curriculum in Toxicology
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina
Soc Mex de Pediatria
Mexico City, Mexico
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