MEXICO CITY (CNN) -- A thick smog covered Mexico City on Thursday, and haze from forest and brush fires blanketed the rest of the nation and caused widespread illness and financial damage.

The longest running pollution alert period since 1996 was extended after officials banned two-fifths of all Mexico City's cars from the streets for the fourth straight day.

Hospitals have reported sharp increases in the number of breathing difficulty cases, as well as high numbers of migraine and eye irritation cases.

Ozone pollution at 3 p.m. registered at 194 on Mexico City's air pollution index, almost twice the level considered safe. A light rain fell on the city Thursday night, but health officials say it is too early to tell if it will help clear the smog.

Schools banned outside activities and residents were advised to stay indoors. Despite the warnings, health problems have continued to increase over the last ten days.

Dozens of flights have been cancelled because of low visibility. The Oaxaca airport in southern Mexico was closed for the fourth consecutive day Thursday.

Hundreds of fires have been burning out of control across much of Mexico and Central America in recent weeks. The haze from the fires has caused health problems throughout the region and as far north as the central United States. In Mexico City, often considered one of the world's most polluted cities, the haze from the fires has thickened the city's infamous smog.

Many of the fires were set by small farmers burning stubble to prepare their fields, as they have for hundreds of years. Hot dry weather caused by El Nino has fueled the fires, making them difficult to fight.

Farmers expect to see much lower yields of basic grains and ranchers have been forced to slaughter their animals before they die of starvation and thirst. The Business Coordinating Council, a private research group, estimates losses in production and wages in Mexico City at $30 million per day since Monday, when emergency measures were put in place

Light and moderately strong rains fell Tuesday and Wednesday in some of Mexico's 31 states, bringing minor relief to some citizens. Major fires continued to burn, however.

In the United States, citizens in Texas were breathing a little easier Thursday after a change in the weather pattern kept much of the smoke from drifting north.

Texas officials cancelled a health alert that called for people to stay indoors. Officials also said the smoke could return next week if the fires are not extinguished, as winds are likely to swing back from the south. U.S. firefighters have been helping Mexican crews battle the blazes since last week.

The Associated Press and Reuters contributed to this report.