Insecticide

man, this all depends on the nature and dosage of the insecticide used.

different insecticide has different nature and toxicity. some are poisonous to only insects, some to all living things.

different insecticide has different mechanism of killing insects (and humans). some by paralyzing the muscles, leaving the victim unable to breath; while some may interrupt the metabolic reactions inside cells.

dosage also affects. when the amount consumed is small, symptoms may not be pronounced. if the body manages to metabolize/excrete the poisons, the effect may appear to be lighter or later.

different people have different tolerance and threshold. threshold is the least amount of substance required to produce effects. examples are 味精MSG and 鎳nickel: someone can eat a lot of MSG without feeling sick and can touch stainless still; while some others feel very ill even taste a little MSG.

you can tell the toxicity by looking at various parameters of insecticide, such as LD50. LD50, or 50%-lethal dose, is the dosage of that substance required to kill 50% of the tested population. of course it's not possible to test on human, but it suggests some information.

another measure is concentration in units like mg/kg body weight. that's about tolerance. exposure time is the third measure that i can think of at this moment.

there're tons of measures to tell whether a substance is dangerous or not. but at first instance you've to specify which insecticide you're talking about.