Mexico City Air Quality Index and Environmental Contingencies

Mexico City reports air pollution on the national Aire y Salud index, which has five levels: good, acceptable, bad, very bad and extremely bad. When ozone or particle levels reach the very bad range, the Megalopolis Environmental Commission (CAMe) declares an environmental contingency (contingencia ambiental), which adds driving bans and health warnings until the air clears.

How does the Aire y Salud index work?

The index turns hourly readings of each pollutant into one color-coded category and a matching health risk. The city’s air monitoring office measures ozone (O3), sulfur dioxide, carbon monoxide, nitrogen dioxide and fine particles (PM10 and PM2.5) at stations across the metro area.

The categories come from the federal standard NOM-172-SEMARNAT-2023, which replaced the 2019 version. The numeric cutoffs differ by pollutant, so the category is a better guide than any single number.

Where can you check live readings?

The official source is the Mexico City air monitoring site (aire.cdmx.gob.mx), run by the Environment Ministry (SEDEMA). It shows an hourly report by station and borough, a forecast for the next 24 hours and the UV index. The same data is in SEDEMA’s free app, called Aire, for iOS and Android, on the @Aire_CDMX account on X, and by phone at 55 5278 9931 ext. 1.

Check the station closest to where you will be, not only the citywide figure. Pollution forms in the center and north and is carried south by the wind, according to the monitoring office, so readings can differ sharply between neighborhoods on the same afternoon.

When is the air usually worst?

Ozone season usually runs from the second half of February until the rains arrive in June, according to the city’s monitoring office. Hot, sunny and still days in that period produce most ozone contingencies, with peaks between 1 p.m. and 7 p.m. In the dry winter months, particles reach the highest concentrations, and thermal inversions, when still air traps pollution near the ground, make episodes worse. The rainy season, June to September, generally brings cleaner air.

Contingencies can still happen outside ozone season. CAMe declared a Phase I ozone contingency in September 2026, for example.

What is a Phase I or Phase II contingency?

A contingency is an emergency program with two levels. CAMe announces activation and suspension in numbered bulletins (comunicados), usually in the afternoon, and the measures start the next morning.

• Phase I is declared when a pollutant reaches the very bad range. In the April 2026 ozone activation, CAMe cited a peak of 163 parts per billion of ozone at one station.
• Phase II is the higher level, for more severe episodes, and its bulletin lists its own, wider set of measures.

Each bulletin lists exactly which vehicles stop the next day, so read the current one on CAMe’s official page rather than relying on last time’s rules.

What do contingencies mean for driving?

A contingency extends the normal Hoy No Circula driving ban, which is why people call it a “double” Hoy No Circula. Under the Phase I bulletin of April 25, 2026, private cars stopped from 5:00 to 22:00 the next day as follows:

1. All cars with hologram 2.
2. Cars with hologram 1 whose plates end in an even number (2, 4, 6, 8, 0) that day; the parity changes with each activation.
3. Cars with hologram 0 or 00 with a yellow sticker and plates ending in 5 or 6; the color and digits also change with each activation.

Exempt vehicles included electric and hybrid cars, emergency services, school transport and vehicles of people with disabilities with the right permit, and motorcycles were exempt in Phase I. Taxis could run from 5:00 to 10:00 on their restricted day. The bulletin also asked residents to fill fuel tanks before 10:00 or after 18:00 and to avoid aerosols and solvent-based paints.

How should you protect your health on a bad air day?

Limit time outdoors during the afternoon peak, especially if you belong to a sensitive group. The April 2026 bulletin advised avoiding outdoor exercise and civic, cultural and recreational activities between 13:00 and 19:00, and it asked organizers to postpone outdoor events in that window.

• Children, older adults, pregnant women and people with heart or lung conditions are the most sensitive groups named in the bulletins.
• If you run or cycle, the monitoring office suggests low-intensity activity on polluted days. In the south, mornings are usually cleaner; in the north, late afternoon.
• People with asthma should agree on a plan with their doctor and check the forecast daily.

Pollution and the city’s altitude of about 2,240 meters can add up for new arrivals. Our guide to altitude sickness in Mexico City covers what to expect in the first days.

Bottom line

Install the Aire app and turn on alerts, and check the station nearest your home before afternoon runs from February to June. If you drive, read the CAMe bulletin each evening during a contingency, because the list of restricted plates changes. This article is general information, not medical advice; ask a doctor how air pollution affects your own condition.

FAQ

Do contingencies close schools or offices?

Bulletins usually ask schools and organizers to cancel outdoor activities during the afternoon peak and encourage remote work. Closures depend on each institution and on the phase, so check your school’s or employer’s notices.

Does a contingency apply to cars with foreign or other-state plates?

Yes. Bulletins apply to vehicles registered anywhere, and the restrictions depend on the hologram the car carries, if any. Cars without a valid Megalopolis hologram are treated like hologram 2 under normal rules.

Is public transport free during a contingency?

Not automatically. Any fare change or free service is announced in the bulletin or by the transit operator, so check the current bulletin before you travel.