SAN FRANCISCO At the American Chemical Society meeting, earlier this week, I stayed at a hotel that fronted onto the kitchen door of a Burger King. That grungy portal was locked most of the time, but explained the source of the beefy scent that perfumed the air from mid-morning on – the restaurant’s exhaust of smoke and meat-derived aerosols. A study presented at the ACS meeting confirmed what my nose observed: that commercial grilling can release relatively huge amounts of pollutants.
However, it found, that’s not a given. Some grills belch out clouds of sooty pollutants. Others are relatively clean, at least in terms of the overall mass they emit. The composition of particles disgorged by different grilling systems also varies quite a bit, the new data show, as does the size of particles that form.
It’s hardly the first investigation to probe meaty pollution. Back in ’91 I had a cover story, “Cholesterol – Up in Smoke: Cooking meat dirties the air more than most people realize.” It focused on research that inferred meat grilling’s share of airborne soot, hydrocarbons, pesticides and other pollutants based on a marker compound: cholesterol.
Caltech scientists assayed what pollutants (including cholesterol) came out of restaurant exhaust hoods and in what relative proportions. Then they parsed out – on the basis of emitted airborne cholesterol – the overall share of combustion pollutants wafting over urban areas that must have been contributed by cooking meat. Their goal was to quantify grilling’s contribution to Los Angeles’ visibility-robbing haze. And those data suggested that meat smoke could be the leading source of fine organic particles – hazy aerosols two microns or smaller – in urban air.
University of Minnesota researchers have now taken a different approach to studying grilling pollution and for a somewhat different reason. Under contract to the American Society of Heating, Refrigerating and Air-Conditioning Engineers, better known as ASHRAE, they measured the relative pollutant output of different grilling technologies and ovens. They focused on the most common systems used in American restaurants.
At ACS, they reported measurements for five of these: a gas-fired conveyor-system pizza oven, two gas-fired broilers, an electric griddle and mesquite-charcoal-fueled broiler. And “in terms of total mass of particles emitted, the charcoal one won,” notes Deborah Gross of Carleton College in Northfield, who worked with the Minnesota team while on sabbatical. It coughed out 35 pounds of particles per 1,000 pounds of burgers grilled. Some 13.5 pounds of that was caught by the exhaust system, leaving a still hefty amount to pollute the air.
Some restaurants employ a conveyor broiler, essentially a moving grate that carries hamburgers past a flame until they’re done. This system spewed not quite a third as much pollution as the charcoal grill. Still, eight pounds per half-ton of meat cooked would have emerged from their exhaust system into urban air. The cleanest griller was the electric clamshell griddle that released only about a quarter pound per half-ton of grilled meat into the exhaust system. And it’s baffled filtering system screened out about half of that pollution before it was released into the air.
The pizza oven proved the cleanest system of all. It released a mere half-ounce of pollutant particles per 1,000 pounds of pepperoni pizza baked. And steak lovers, take heart. Grilling this meat in a gas-fired broiler released only a little over a pound of particles per 1,000 pounds of meat, of which only some three-quarters of a pound escaped the exhaust system.
Not surprisingly, the charcoal grilling and conveyor driven gas broilers for cooking burgers proved the sootiest. They also released the largest size polycyclic aromatic hydrocarbons, or PAH, molecules. A number of PAHs, such as benzo[a]pyrene, are known or suspected carcinogens. Especially the bigger PAHs, Gross notes. And several of these big PAHs, like benzo[a]pyrene, were among those spewed by the higher-polluting burger grills – but not the others.
But the cleanest of the burger systems also, overall, produced the smallest particles – some only 50 billionths of a meter in diameter (sorry, the scientists used SI units for some measurements, as here, and standard American units for others). Those teensy particles are the size most likely to escape vent filters and ultimately waft long distances. If inhaled, they’re also of a size likely to become lodged most deeply in the lungs – or even to move into the bloodstream and brain (look for more on that soon in a print story).
All of these data are useful for restaurants that may need to retrofit their systems with pollution controls in the near future. But there’s a take-home for the rest of us who enjoy barbecuing all summer. The meaty aroma emanating from our grills comes from particles that may seed the neighborhood and beyond with traces of not-so-nice pollutants. And it reinforces again why we should always turn on the range hood when cooking or broiling to limit the amount of these potentially long-airborne pollutants that seed our indoor air.