If we were talking about ordinary numbers, you'd be right. But today I'm talking biology and the synergistic effects of toxins.
The term we need to add is lethal dose (LD). A LD50 means the 50% of the test subjects (usually lab mice) die. A LD1 means that just 1% die.
But the synergistic effect of two or more toxins is not simple math. A 1978 paper discusses how a LD1 dosage of mercury when combined with a LD1 dosage of lead resulted in the death of 100% of the lab rats - LD100.
LD1 + LD1 = LD100
Of course the dosage for humans does not directly translate from animal studies and it's frowned upon to perform such lab tests on humans (though we're guinea pigs for what the various governments and corporations allow/force upon us).
There are also several levels below death. Sometimes damage is permanent. Sometimes it's reversible. Some toxins are cumulative.
But the take-away point is that when you read about the results of safety studies - the individual substance may be as "safe" as advertised, but when combined with other ingredients - the results can be damaging to your health or even result in death.
I've included a couple of articles for your reading "pleasure":
Dr Miller discusses effects including mercury and aluminum.
Another discussion on synergistic toxicity.