Question
Are apple seeds poisonous?
Answer
Mostly no.
Explanation
Apple seeds contain a compound (amygdalin) which can produce hydrogen cyanide when chewed. Hydrogen cyanide is poisonous and can be fatal to humans even at a small dose, but it would require an unusual number of apple seeds to be ingested.
Per gram of well-chewed or finely ground apple seeds the human body could metabolise 0.06-0.2 mg of cyanide. Severe and potentially lethal cyanide poisoning would occur on consuming 0.5-3.5 mg of cyanide per kilogram of body weight.
The lethal dose for an individual weighing 73 kg would therefore be about 183-4,258 grams of apple seeds. Even a toddler of 9 kg could expect to survive at less than 23-525 grams of apple seeds.
Sources
Bolarinwa, I.F., Determination of amygdalin in apple seeds, fresh apples and processed apple juices (Food Chemistry, Volume 170, 2015)