Carbon monoxide-treated fish is generally safe to eat in terms of the gas itself. The trace amounts of carbon monoxide used in packaging don’t pose a direct health risk, and the U.S. FDA has not objected to its use. The real concern isn’t the carbon monoxide. It’s that the treatment can make old fish look fresh, potentially hiding spoilage that could make you sick.
Why Fish Is Treated With Carbon Monoxide
Carbon monoxide binds to myoglobin, the protein that gives fish flesh its color. In untreated tuna, for example, myoglobin gradually oxidizes when exposed to air, turning the flesh from red to brown over a few days. Carbon monoxide locks myoglobin into a stable, bright red form and prevents this natural color change. It also slows fat oxidation, the chemical process that causes rancidity, which extends shelf life.
The treatment is most common in tuna, but it’s also used in tilapia, salmon, and other species. Fish is typically exposed to low concentrations of carbon monoxide (around 0.4% in the packaging atmosphere) either as a gas flush in modified atmosphere packaging or through “tasteless smoke,” a filtered wood smoke that delivers carbon monoxide without adding flavor.
The Actual Safety Concern: Hidden Spoilage
Carbon monoxide itself isn’t toxic at the levels used in fish packaging. The problem is what the cosmetic effect conceals. Normally, you can judge freshness partly by color. A piece of tuna turning brown is a visual signal that it’s aging. CO-treated fish retains an artificial “watermelon red” appearance indefinitely, even as the flesh deteriorates in ways you can’t see.
This matters most for fish species prone to histamine buildup. Tuna, mackerel, mahi-mahi, and other dark-fleshed fish naturally contain an amino acid called histidine. When bacteria grow on these fish (due to poor temperature control, for instance), they convert histidine into histamine. Eating fish with high histamine levels causes scombroid poisoning, a reaction that mimics a severe allergic response: flushing, headache, nausea, and sometimes difficulty breathing. Cooking doesn’t destroy histamine once it has formed.
With untreated fish, browning and off-odors would tip you off before histamine reached dangerous levels. CO-treated fish can look perfectly fresh while bacteria quietly produce histamine underneath that locked-in red color. The European Commission has specifically cited this increased risk of histamine exposure as a public health concern with CO treatment.
How the U.S. and EU Regulate It Differently
In the United States, carbon monoxide treatment in fish operates under a legal gray area. Companies have submitted notifications to the FDA asserting that CO is “generally recognized as safe” (GRAS), and the FDA has responded that it has “no questions” about those claims. But this isn’t the same as formal approval. As a Congressional Research Service analysis noted, the FDA’s lack of questions doesn’t mean the agency has actually determined CO to be safe. It means the agency hasn’t challenged the manufacturer’s own conclusion. The practical result: CO-treated fish is widely sold in the U.S.
The European Union takes a stricter stance. EU regulations explicitly prohibit treating unprocessed fish with carbon monoxide. The rationale is straightforward: additives intended to change a fish’s color mislead consumers about freshness, and that deception creates a downstream food safety risk. Japan and Canada have also restricted or banned the practice.
Labeling: What to Look For
In the U.S., CO-treated fish must be labeled as containing a chemical preservative under federal food labeling rules. You’ll typically see “treated with carbon monoxide” or “treated with tasteless smoke” on the packaging. The requirement applies to retail packaging, so if you’re buying from a fish counter where the product has been removed from its original wrapper, this information may not be visible. Asking your fishmonger directly is the most reliable way to find out.
How to Spot CO-Treated Fish
The most telling sign is color that looks too vivid and too uniform. Fresh, untreated tuna has a deep, slightly translucent red that naturally varies across the fillet, and it begins browning within a day or two of being cut. CO-treated tuna holds an unnaturally bright, almost neon watermelon-red color that stays consistent across the entire surface and doesn’t change over time. If a piece of tuna in the display case looks startlingly red and perfectly even in color, it has likely been treated.
Smell is a more reliable freshness indicator than color for treated fish. Fresh fish should smell like clean ocean water, not “fishy.” If CO-treated fish smells off, trust your nose over your eyes.
Does the Treatment Affect Nutrition?
Carbon monoxide’s interaction with fish is primarily cosmetic and chemical at the surface level. It binds to pigment proteins and slows fat oxidation, but it doesn’t alter the protein structure, omega-3 fatty acid content, or vitamin profile of the fish in any meaningful way. The nutritional value of a CO-treated fillet is essentially the same as an untreated one of equivalent freshness. The key word there is “equivalent freshness,” because a CO-treated fillet that’s been sitting longer than it should may have degraded nutritionally in ways that have nothing to do with the gas and everything to do with age.
Practical Tips for Buying Safely
- Check the label. Look for “carbon monoxide” or “tasteless smoke” on packaging. If you see it, the fish isn’t necessarily bad, but you’ll need to rely on other freshness cues beyond color.
- Use your nose. Any sour, ammonia-like, or strongly fishy smell means the fish is past its prime, regardless of how red it looks.
- Press the flesh. Fresh fish springs back when you press it with a finger. If it stays indented, it’s deteriorating.
- Mind the temperature. Histamine-producing bacteria thrive in warm conditions. Make sure the fish you’re buying is properly chilled (below 40°F), and refrigerate it promptly. This is the single most important factor in preventing scombroid poisoning, whether the fish is CO-treated or not.
- Buy from reputable sources. Vendors with high turnover are less likely to have fish sitting around long enough for hidden spoilage to become a problem.