Comparing Measurements with Standard Limits
In Mexico City, an environmental contingency for ozone is sometimes announced, and people are asked to avoid outdoor activities. In a rural community, a water test of a well may show that it is not safe to drink. How do we decide that a value is already "too high"? By comparing it with a limit set in a standard. In this page you will learn how to make that comparison and turn it into a recommendation.
The limits in the examples and exercises are sample figures, plausible for practice. For a real decision, you need to consult the current standard.
What a standard's limit is
A standard is an official document that sets the maximum value of a pollutant that is considered acceptable to protect health. That value is the limit. It is measured in the same units as concentration: for example, ppm.
A limit always states three things: which substance (ozone, arsenic…), where (air, water, soil), and under what conditions (for example, "8-hour average"). If any of these is missing, the comparison makes no sense.
Remember the basics of ppm: in water, $1\ \text{ppm} = 1\ \text{mg/L}$; in soil, $1\ \text{ppm} = 1\ \text{mg/kg}$; in air, ppm by volume.
Comparing: is it exceeded or not?
To compare, both values must be in the same unit. Then:
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If $\text{measured} \le \text{limit}$: the standard is met.
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If $\text{measured} > \text{limit}$: the limit is exceeded.
When ozone is high, authorities usually recommend avoiding intense outdoor exercise (especially during the sunniest hours), keeping children and older adults indoors, and driving less. Ozone irritates the respiratory tract, and people who exercise breathe in more air and more pollutant.
An air quality standard sets the ozone limit at 0.095 ppm as an 8-hour average. A monitoring station in Mexico City records 0.070 ppm on Monday and 0.120 ppm on Tuesday.…
An air quality standard sets the ozone limit at 0.095 ppm as an 8-hour average. A monitoring station in Mexico City records 0.070 ppm on Monday and 0.120 ppm on Tuesday. State on which day the limit is exceeded and explain one measure that authorities usually recommend to the population when ozone is high.
How many times it is exceeded
Knowing that the limit is exceeded is not enough: by how much matters. To find out, divide:
$$\text{times exceeded} = \frac{\text{measured concentration}}{\text{limit}}$$If the result is 1, the value is right at the limit. If it is 2, there is double the allowed amount. If it is less than 1, the standard is met. It is a ratio, so it has no units.
Converting units before comparing
Sometimes the limit and the measurement come in different units. Before comparing, you have to bring them to a single unit. In water it is very convenient:
$$1\ \text{ppm} = 1\ \text{mg/L}$$So, $0.01\ \text{ppm} = 0.01\ \text{mg/L}$. If a report gives $\mu\text{g/L}$, remember that $1\ \text{mg} = 1{,}000\ \mu\text{g}$, so $0.01\ \text{mg/L} = 10\ \mu\text{g/L}$.
From data to decision
A recommendation for the authorities or the community has four parts: data, comparison, conclusion, and a justified proposal.
A drinking water standard sets the arsenic limit at 0.01 ppm. The analysis of a community's well reports 0.018 ppm of arsenic. Express both values in $\text{mg/L}$,…
A drinking water standard sets the arsenic limit at 0.01 ppm. The analysis of a community's well reports 0.018 ppm of arsenic. Express both values in $\text{mg/L}$, calculate how many times the limit is exceeded, and write a short recommendation for the authorities: can that water still be used for drinking? What options would you propose (treatment, another source, mixing with clean water)? Justify your proposal with the data.
Back to the beginning: a "high" value is decided by comparing it with the limit. If ozone goes past the limit, there is a contingency; if arsenic goes past the limit, the water should not be drunk.