Atlas de l'exposition aux pollutions atmosphérique et sonore


Boxplot and violin graph

A box and whiskers plot, or boxplot (rectangle in the image below), is used to analyze the distribution of a variable. The rectangle is constructed based on the first and third quartiles (25e et 75e centiles) of the distribution; the central bar in the rectangle represents the median (50e centile). The extremities of the whiskers indicate the maximal and minimal values. The larger the boxplot is, the more dispersed the variable is.
The violin graph (in grey in the image below) adds additional information to that shown in the boxplot. It is a reverse histogram for the variable studied. A point thus corresponds to a series of observations grouped around the same value. In the example below concerning the physical effort involved in public transport, one can see two different modes (dominant values), with the first located clearly above the median and the second below. This highlights two different situations. The first is characterized by a grouping of strong values and corresponds to moments of activity (walking, stairs, etc.). The second is characterized by weak values, and corresponds to moments when the participants were seated in the subway or bus.


Noise is measured with a Brüel & Kjaer dosimeter (type 4448), which refers to an average measurement of noise in decibels (dB(A)) for each minute of the trip (LAeq 1 min). The World Health Organization (WHO) recommends that the 55 dB(A) outdoor noise guideline value not be exceeded during the day. In addition, the Quebec Ministry of Transport’s policy on road traffic noise recommends that noise not exceed 65 dB(A) along traffic lanes.

 Heart rate

This value corresponds to the number of heartbeats per minute.


The inhalation value corresponds to the dose of the NO2 pollutant inhaled during the trip in taking ventilation into account (number of litres of air inhaled per minute).

 Metabolic Equivalent of Task (MET)

This measure allows one to evaluate the intensity of a physical activity. It ranges from 0.9 (sleep) to more than 18 (running at 17.5 km/h or more). There are several MET scales that distinguish between several levels of physical activity, especially:

  • low-intensity physical activity (MET under 3);
  • moderate-intensity physical activity (MET from 3 to 6);
  • high-intensity physical activity (MET above 6).

 Air pollution (NO2)

Air pollution was measured using a portable Aeroqual 500 device with nitrogen dioxide (NO2), temperature, and humidity sensors. The NO2 pollutant was chosen as its emissions mainly come from the engines of motor vehicles. According to the World Health Organization (WHO), the air quality guideline value that should not be exceeded is 200 μg/m3, over short durations (one hour). Beyond this threshold, “it ⦋NO2⦌ is a toxic gas which causes significant inflammation of the airways” (WHO).