A subsequent section of the code is more specific with regard to nitrous and motor vehicles. According to Ohio Revised Code 2925.33, "Possessing nitrous oxide in motor vehicle," states that "no person shall possess an open cartridge of nitrous oxide ... [w]hile operating or being a passenger passenger in or on a motor vehicle on a street, highway, or other public or private property open to the public for purposes of vehicular traffic or parking," or "[w]hile being in or on a stationary motor vehicle" in such a situation. Violators are guilty of possessing nitrous oxide in a motor vehicle, which is a misdemeanor of the fourth degree. While exceptions are given for dentists, pharmacists, emergency medical services, veterinarians, and physicians, none is given for the general public or for use as an automobile engine additive. Presumably, this law is aimed at drug-abusing punks who might inhale nitrous and engage in other mischief whilst in their cars, but its vague wording unfortunately opens it up to applying to nitrous oxide systems as we enthusiasts know them.
Pennsylvania
Exhaust Systems
Pennsylvania Consolidated Statutes, itle 75 (Vehicles), 4523, states in subsection (a) that: "Every motor vehicle operated on a highway shall be constructed, equipped, maintained and operated so as not to exceed the sound level for the vehicle as prescribed in regulations promulgated by the department." Further, subsection (c) says "[e]very motor vehicle shall be equipped with a muffler or other effective noise suppressing system in good working order and in constant operation and no muffler or exhaust system shall be equipped with a cutout, bypass or similar device." In addition, subsection (d) states that "[n]o person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the motor of the vehicle above the maximum levels permitted under subsection (a) or violate the provisions of subsection (b)" (where (b) refers to exhaust entering the passenger compartment). Interestingly, it goes on to say, "[h]eaders and side exhausts are permitted provided the vehicle meets all the requirements of this section." The vague wording here makes intent really tough to gauge, and you probably shouldn't take this to say that open headers are legal in Pennsylvania!
You can find basically the same thing in the Pennsylvania Code, Title 67 (Transportation), 175.75, though it adds: "The exhaust system of a vehicle may not be modified in a manner which will amplify or increase noise emitted by the motor of a vehicle above the maximum level permitted by Chapter 157 (relating to established sound levels)." That Chapter does enumerate decibel noise limits; but here again, they are for vehicles in motion, so noise from tires and other sources factor in.
Nitrous Systems
Pennsylvania Consolidated Statutes, Title 18 (Crimes and Offenses), 7303, relates to the sale or illegal use of certain solvents and noxious substances. Nitrous oxide is defined as falling under the stated category of a noxious substance or substance containing a solvent having the property of releasing toxic vapors or fumes." Sale or possession is a misdemeanor of the third degree, though fortunately, this seems to apply only if one "for the purpose of causing a condition of intoxication, inebriation, excitement, stupefaction, or the dulling of his brain or nervous system, intentionally smell[s] or inhale[s] the fumes," or sells it to someone likely to use it for that purpose. Again, it would seem the legislative intent here was not to prohibit nitrous use in car engines, especially since smelling or inhaling is specifically mentioned. However, Pennsylvania may have a nitrous permit system we don't know about, so you should do some research on this topic too.