By Mica Rosenberg MEXICO CITY – Chronic pollution in Mexico City, which stains the sky yellow and can trigger government warnings to stay indoors, could be killing off residents' sense of smell, scientists say.

Tests showed people in Mexico City – a sprawling metropolis crammed with around 20 million people and 4 million cars – struggled to sniff out everyday odors like coffee and orange juice compared to residents of a nearby town.

Their noses are so badly damaged from a life inhaling toxic particles that they also find it harder to detect the scent of rotten food, said researcher Robyn Hudson who ran the study.

“We added a substance (to powdered milk) that is a common contaminant of food, something that smells disgusting basically – like a sour, rotting cabbage,” said Hudson.

“We were able to see at what point ... they would start to reject the contaminated sample, say 'ew yuck! no! take it away please,'” said Hudson, an Australian and a senior research scientist at Mexico's National Autonomous University (UNAM).

Mexico City is one of the world's most polluted capitals, along with Beijing, blighted by its thin high-altitude air and a ring of surrounding mountains that trap exhaust fumes belched from smoky buses and factories on the city outskirts.

Contamination levels are better now than two decades ago, but from a high-story window it's still hard most days to make out the snow-capped peaks that surround the city through the murky shroud of brownish smog.

Mexico City's ozone levels exceed World Health Organization standards on approximately 300 days of the year.

VULNERABLE CELLS

Hudson's team compared Mexico City dwellers with a group from the nearby rural state of Tlaxcala, and found country folk could detect contaminants at lower concentrations.

The researchers are now testing to see whether pollution can cause more serious cellular damage in the nose and eyes.

On entering the nose, odors pass through a thin layer of mucus where they are detected by sensory cells which send the brain messages that something reeks or smells rosy. The cells are vulnerable to damage from pollutants, Hudson said.

“The olfactory receptors are very exposed,” she said. “They are just hanging out there in the mucus.”

Hudson suspects pollution could also affect taste, given the two senses are intimately linked.

The loss of smell is one of a gamut of problems caused by Mexico City's smog, which prompts a handful of warnings a year for residents to avoid exercising or hanging about outside.

Health experts worry about lung infections, asthma, heart attacks and cancers. A 2007 study in the “American Journal of Respiratory and Critical Care Medicine” found Mexico City school children had unusually small lungs.

The problem is aggravated by the fact Mexico City stands some 7,400 feet (2,240 meters) above sea level in a sunken lake bed rimmed by mountains. Its thin atmosphere and bowl-shaped valley concentrates pollutants within the city.

City authorities have worked to remove the worst-polluting vehicles from the road, but as the capital's population grows it buys up to 250,000 new cars each year.

Asthma, allergies and other respiratory problems are common among residents, whose grimy windows need weekly scraping.

A 10 percent drop in toxic air particles could save 3,000 lives a year and prevent 10,000 cases of chronic bronchitis, says pollution watchdog The Blacksmith Institute.

“I know I'm inhaling poison,” said 38-year-old Guadalupe, a candy seller at a busy Mexico City road intersection with cars whizzing by. “But there is nothing I can do.”

(Additional reporting by Anahi Rama; Editing by Catherine Bremer)