I hate to say these things and sound like a conspiracy theorist but once you wake up to the signs, then you can safeguard. One ingredient that the use a lot is chemicals from exterminators. These chemicals tend to cause insects to suffocate to death or die by drying them out. Applied to humans the can cause equal debilitating damage too. Example
Symptoms of arsenic poisoning may include:
red or swollen skin.
skin changes, such as new warts or lesions.
abdominal pain.
nausea and vomiting.
diarrhea.
abnormal heart rhythm.
muscle cramps.
tingling of fingers and toes.
It is a dead give away, for example, when you see in the dead of winter, an exterminator truck show up at a bordering neighbor's house twice in the beginning and end of January and someone in your family have these symptoms.