Clients who find spiders in and around their house often call to ask if the spider is "poisonous". Without even seeing the spider in question the answer is always "no." There are no poisonous spiders. However, as far as I know, all spiders are venomous.
The terms poisonous and venomous cause confusion primarily because most people use the two terms interchangeably - - - especially when talking about spiders. I have always found it somewhat amusing that people never seem to ask if there are any poisonous bees or wasps, but commonly do so when discussing spiders.
The distinction between poisonous and venomous is how the substance (poison or venom) is administered or delivered. A poison is a substance that causes damage, illness or death after being ingested or absorbed through the skin (harmful to eat, breathe or touch). Venom, on the other hand, is a toxin that is injected in the process of a bite or a sting. Thus, venomous snakes inject venom through their fangs, venomous bees, wasps and hornets inject venom through their "stingers" and venomous spiders inject venom through their chelicerae (mouthparts). The purpose of spider venom is to subdue the prey, generally small insects that must be immobilized prior to being eaten.
A better distinction than to ask if spiders are poisonous is to ask if they are potentially dangerous or potentially medically important. For Iowans the answer is almost always still, no. There are a very small number of spiders that might cause a localized pain if they could be coaxed to bite (the pain would be similar to that of a mild bee sting).
There are only two spiders, both rarely found in Iowa that have venom capable of causing illness in humans. The black widow spider venom is a neurotoxin that can cause headache, dizziness, shortness of breath and other systemic symptoms. Death is very rare. The brown recluse spider bite is usually painless. If sufficient venom is injected a skin ulcer may develop at the bite site.
Recent newspaper articles have commented on the common misdiagnosis of spider bites by medical professionals. A common cause of skin irritation and bite-like skin reactions is MRSA, methicillin-resistant Staphylococcus aureus. This strain of staph is resistant to broad-spectrum antibiotics and can cause skin infections that may turn into deep, painful abscesses requiring surgery. The Mayo Clinic recommends seeing your doctor for any skin problem that becomes infected.
Brown recluse spider