C stands for concentration of fibers per cubic centimeter of air. In air sampling, you relate the total number of fibers counted to the volume of air drawn through the sampler, so the result is expressed as fibers per unit air volume (typically fibers per cubic centimeter). This normalization to air volume is what lets you compare results and assess exposure, rather than reporting a raw count, a volume measure, or a count per microscope field.

Understanding how metric volumes relate: a liter is defined as a cubic decimeter, which is a cube 10 centimeters on each side. The volume of that cube is 10 cm × 10 cm × 10 cm = 1000 cubic centimeters. Since 1 cubic centimeter equals 1 milliliter, 1 liter also equals 1000 milliliters, or 1000 cubic centimeters. So there are 1000 cubic centimeters in one liter. The other options describe volumes that don’t match this relationship: 100 cc would be 0.1 liter, 10,000 cc would be 10 liters, and 1 cc per liter represents a much smaller amount.

The idea behind Major Spill Response Level is to flag spills that could pose significant exposure or risk in the area around a home, so responders escalate quickly. For residential areas, the threshold is defined as a spill that covers 32 square feet or runs 50 linear feet. Meeting or exceeding either measurement means the spill is large enough to require formal, rapid actions like containment, notification, and additional resources. Why this size matters: 32 square feet is roughly a 6-by-5 foot patch. That size can contaminate multiple surfaces or soil in a home yard or around entryways, increasing potential exposure. If the spill forms a linear spread along a boundary, a length of about 50 feet can affect a larger stretch of property or a roadway edge, again elevating risk and the need for a coordinated response. The other options represent thresholds that aren’t used for this residential Major level—either they’re far larger (which would push the spill into a different category or a less sensitive context) or much smaller (which would imply a Minor or other lower level).

Fiber Density. This quantity describes how many fibers occupy a given space, which is the direct measure used to quantify how densely fibers are present in a sample or in the air. It’s the most relevant way to express how much fiber exposure someone might encounter, since higher density means more fibers available to be inhaled. The other ideas refer to exposure or concentration concepts that aren’t as tightly tied to the literal fiber count per space, so they don’t fit the symbol E as cleanly in this item.