### Relation heart rate and minute ventilation

The bicycle ergometer tests showed that heart rate and minute ventilation are highly correlated, with substantial differences in regression equations between the individuals.

Minute ventilation during the bicycle rides were on average 2.1 times higher than in the car (individual range from 1.3 to 5.3) and 2.0 times higher than in the bus (individual range from 1.3 to 5.1). The ratio of minute ventilation during cycling compared to in bus or car was higher in women than in men.

Using overall equations for men and women instead of the individual coefficients resulted in good prediction of the mean minute ventilation levels of the population, but resulted in substantial differences in estimated minute ventilation on the individual level. Consequently, for studies on group level the use of overall equations, for instance obtained from a sample of the full study population, can be justified. However, when looking at individual level, the use of individual regression coefficients provides more precise data.

The correlation between heart rate and minute ventilation was high (average R^{2} 0.90) in the present study, as has also been shown before [12]. The kind of activity (upper-body activity or lower body activity) influences the relation between minute ventilation and heart rate [12]. In our study the use of bicycle ergometers is therefore appropriate for estimating the minute ventilation during cycling. Heart rate is not only influenced by exercise, but also by emotions, coffee, drugs, time of the day, temperature. These factors probably did not play an important role in this study.

The assumption of a log-linear relationship between heart rate and minute ventilation as used in this study, was based upon other publications [11–14]. The scatter plots (three of them presented in figure 2) and R^{2} values of our 34 tests confirmed the good fit. Others have assumed a linear relation with one or two break points at the ventilatory compensation point (VCP) and/or the lactation threshold (LT) [15]. We did not have information about VCP or LT. Since we did not measure the full range of heart rate (up to maximum), we had limited possibilities to assess the shape of the relationship in our own data.

Other studies have used the oxygen uptake rate to estimate minute ventilation [16]. However, measuring oxygen uptake during commuting is difficult and would influence air pollution inhalation, therefore this method was not feasible in this study.

As shown in table 6, the average regression equations of heart rate and minute ventilation (natural log transformed) found in this study, were quite similar to that found by Samet [12] but differed somewhat from that of Colucci [13]. Individual slopes and intercepts differ considerably, as has also been reported before [12].

Applying the coefficients from Samet and overall equations from our own study instead of the individual coefficients, resulted in good prediction of the minute ventilation during cycling and of the mean ratio of minute ventilation of cyclists compared to car passengers. Using the coefficients from Colucci did not result in good predictions. However, details of this study, such as age of the test persons and number of persons, are missing, so we cannot estimate whether the study groups are truly comparable or not. The good results from the use of the average coefficients of Samet and our own average coefficients leads us to conclude that for studies on group level the use of overall equations can be justified. Large studies could estimate the relation between heart rate and minute ventilation for a sample of their population and apply it to the full population when only looking at mean group values.

The need for using individual slopes and intercepts instead of mean values in assessing minute ventilation levels at the individual level is underlined by figures 3 and 4. Minute ventilation levels during cycling and ratios of minute ventilation of cyclists to car passengers can differ widely when calculated using the mean regression coefficients instead of using the individual coefficients. We therefore conclude that for studies on individuals it is necessary to determine the individual relation between heart rate and minute ventilation, in agreement with previous studies [11, 12, 17].

### Minute ventilation levels during commuting

The volunteers did not drive the car in the study, but were seated in the back. The heart rates and relation between heart rate and minute ventilation in the car are therefore not much influenced by stress from traffic participation. The slightly higher heart rates in bus compared to car may be caused by more space in the bus to move around, though the volunteers kept seated. The US-EPA reported that minute ventilation levels of car drivers were only 10% higher compared to minute ventilation levels of car passengers [18]. The mean minute ventilation of car passengers in our study was 11.8 l/min, this is in line with the 12.3 l/min that has been measured in car drivers before [10].

The speed of the cyclists in the study was limited because of the limited speed of the technician cycling the bicycle loaded with heavy air monitoring equipment on partially hilly stretches. The average speed during the study was 12 km/h, while the average speed of cyclists in Dutch cities is estimated to be around 15 km/h, including stops while waiting for traffic lights. The differences in minute ventilation between cyclists and car and bus passengers are for most people therefore higher during cycling in everyday life than as determined in this study. In our study, minute ventilation of cyclists was on average 2.1 times higher than of car passengers, this is slightly lower than the factor 2.3 as has been calculated in a study where the speed of cyclists was not hampered [10]. In agreement, the mean minute ventilation of cyclists measured in the mentioned study [10] was 29.1 l/min while in our study the mean minute ventilation was 23.5 l/min. In a study on five young (20-32 years), male bicycle messengers, mean ventilation levels during cycling was 31 l/min, and mean heart rate during cycling was 107 bpm [14]. The bicycle messengers can be expected to cycle faster than the average cycling speed, so these data are also in line with our study, where mean minute ventilation of cyclists was 23.5 l/min and the mean heart rate was 100 bpm.

Intake of air pollutants is influenced by minute ventilation, but deposition of air pollutants is also influenced by the amount of nasal and oral breathing and by depth of inhalation. More oral breathing and deeper inhalation will occur during exercise, both leading to higher deposition of pollutants. In a study by Daigle et al [19] a 3.3-fold increase in minute ventilation led to a more than 4.5-fold increase in total ultrafine particle deposition.

In the present study we have not been able to measure oral and nasal breathing separately, nor have we measured the depth of inhalation. The ratios of minute ventilation of cyclists compared to car and bus passengers as calculated in the present study, are for those reasons likely to underestimate the true differences in deposition of air pollution inhaled by cyclists compared to car and bus passengers.

The inhaled dose of air pollutants of different groups of commuters is influenced by minute ventilation. We have shown that minute ventilation levels of cyclists are more than two times higher than commuters using a car or public transport. The increased minute ventilation of cyclists and other physically active commuters should be taken into account when comparing inhalation of air pollutants between different groups of commuters.