MEXICO CITY (AP) — Small-particle pollution spiked in Mexico City to almost six times acceptable limits Sunday, a level not seen for years in the Mexican capital.

Levels of small particles, known as PM10, reached 581 points in the city’s Iztapalapa borough, on a scale where 100 is considered the highest acceptable level.

No formal pollution alert was declared, but the levels were enough to earn a rare “extremely bad” rating from the city’s air-quality monitoring network.

Alerts, which can force some vehicles to stay off the road, are normally declared when pollution reaches between 1.5 and 2 times acceptable limits.

PM10 particles are often from dust or soil kicked into the air, and the network warned that high winds were likely to cause an increase in particle levels.

Ozone and extremely small particles, known as PM2.5, are often triggered by vehicle emissions and are traditionally more of a problem in Mexico City. But they did not appear to play as much of a role in Sunday’s pollution spike.

Spring is dry season in central Mexico and it is also a time when farmers on Mexico City’s outskirts traditionally burn grass and weeds to prepare fields for planting.