Two months after releasing an alarming report on herbicides in cereals and other food marketed to children, an environmental advocacy group is back with additional findings. This time, tests detected trace amounts of glyphosate -- the active ingredient in Monsanto's weed killer Roundup -- in all of the nearly 30 samples of oat-based cereal and snack bars tested.

All of the samples tested within range of limits deemed acceptable by the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration, and there's widespread disagreement about what's an acceptable amount of a chemical termed a "probable carcinogen" by the World Health Organization.

The latest tests by the Environmental Working Group, or EWG, detected glyphosate in all 28 samples of products made with conventionally grown oats. All but two of the 28 samples had levels of glyphosate above EWG's own health benchmark of 160 parts per billion, or ppb.

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The products tested included 10 samples of different types of General Mills' Cheerios and 18 samples of different Quaker brand products, including instant oatmeal, breakfast cereal and snack bars. The highest level of glyphosate found by the lab was 2,837 ppb in Quaker Oatmeal Squares breakfast cereal, the EWI said.

The results released Wednesday follow the results two months ago in which the EWG found glyphosate in all but two of 45 samples of foods made with conventionally grown oats, and in about one-third of the 16 products made with organic oats.

The EWG advocates ridding food of glyphosate, and its scientists recommend limiting daily consumption of the chemical to 0.01 milligrams. For a 60-gram portion of food, that translates to a safety standard of 160 parts per billion, or ppb. The group said its benchmark, far more stringent than that of government regulators, is based on the risks of lifetime exposure, arguing that "small, repeated exposures can add up if someone eats food containing glyphosate every day."

Noting that most crops are grown in fields using some form of pesticides, "trace amounts are found in the majority of food we all eat," General Mills noted in an email to CBS MoneyWatch. "The extremely low levels of pesticide residue cited in recent news reports is a tiny fraction of the amount that the government allows. Consumers are regularly bombarded with alarming headlines, but rarely have the time to weigh the information for themselves."

The EWG report uses s level for glyphosate "that is detached from those that have been established by responsible regulatory bodies in an attempt to grab headlines," Quaker responded in an email. "The Quaker products tested by EWG are safe," the company added.

EWG, however, dismisses federal standards as "often outdated" and "heavily influenced by lobbying from industry."