Particle exposure and inhaled dose while commuting by public transport in Mexico City

Abstract

The time a person spends in daily commutes in large cities may represent a major intake of airborne pollutants. In this context, assessments of the variability in exposure to pollutants across transport modes is therefore essential to formulate efficient policies on public health and urban mobility. In this context the present study investigates which public transport mode would expose a person to the least amount of particle pollution along a representative route in a busy area of Mexico City. A set of portable instruments was used to measure in-situ a number of particle metrics across the transport microenvironments encountered in the available transport modes. Commuters can cover the selected route by taking the subway, the Bus Rapid Transit (BRT) system, hailing a taxi or using a real-time ridesharing service. They have also the option of cycling and walking. Simultaneous measurements were conducted at a nearby park to differentiate the contribution from emission sources along the commuting route from the overall pollution within the urban fabric. The air-conditioned cars of the ridesharing service emerged as the cleanest transport mode, with exposure concentrations below those recorded at the non-road site. The subway registered the highest concentrations of particles, especially ultrafine particles. All transport modes were affected by fumes generated from cooking activities from informal food stalls, which are ubiquitous along Mexico City's streets, particularly at subway stations and busy bus stops. It was found that after considering physical activity and travel time, cyclists and pedestrians received the largest inhaled doses. Commuters breathe in four times more particles walking and cycling, as compared with traveling by ridesharing service. Similarly, commuters who choose to make the journey via BRT and subway reduce their particle intake by a half compared to walking, and those who hail a taxi by a third. Among the evaluated metrics, the number of particles, active surface area and black carbon concentration were the most illustrative parameters to evaluate the particle exposure in the transport environment of Mexico City.