Nitrogen reaches the pool through very everyday, and almost always invisible, routes, there's no single moment where you "notice" it going in. It builds up little by little across the whole season, and its effect is a lot like phosphates, it feeds algae, even though it gets discussed far less often.
The nitrogen that keeps building up
Where nitrates, nitrites and urea come from
These are nitrogen-containing compounds, and they reach the water mainly through these routes:
- Swimmers' sweat and urine: by far the most constant source. Sweat contains urea, and so does urine, every person who swims contributes a small amount, and with the number of swimmers over a season, it adds up.
- Rain and runoff: rainwater picks up nitrates from the atmosphere and from the surfaces it crosses before reaching the pool, roofs, gardens, paths.
- Nearby garden fertilizer: lawn and plant fertilizers are usually nitrogen-rich. If there are landscaped areas right next to the pool, watering or rain can wash fertilizer residue into the pool, especially if the ground slopes toward the water.
- Decomposing organic matter: leaves, insects and other debris that fall into the water also release nitrogen compounds as they break down.
Why they feed algae just like phosphorus does
Algae, like any plant, need nitrogen and phosphorus to grow, they're two of their main nutrients. A lot of attention goes to controlling phosphates, but if nitrogen is available in abundance, algae can still find "food" even when phosphorus is low. That's why it's worth thinking of nitrogen as the other side of the same equation, not a separate problem: cutting down only one of the two nutrients isn't always enough to slow growth if the other one is still available in quantity.
A high level of nitrogen compounds can also raise chlorine demand: part of the disinfectant gets used up reacting with these organic compounds instead of being available to eliminate microorganisms, which explains those seasons where "the chlorine disappears" faster than usual for no obvious reason.
How they build up over a season
Unlike a sudden chlorine or pH spike, nitrates and urea tend to rise gradually and quietly. Every busy swimming day adds a little more; every rainfall adds a little more; garden fertilizer contributes on and off. None of this is visible at a glance, the water can still look clear, until it converges with other factors (low chlorine, high temperature, insufficient filtration) and an algae problem shows up that takes more effort than usual to solve.
How to keep them under control
No complicated treatment is needed, just consistency with a few habits:
- Shower before swimming: a shower of just a few seconds removes a good part of the sweat, sunscreen and skin residue that would otherwise go straight into the water.
- Discourage urinating in the pool: it sounds obvious, but it's the most concentrated and most avoidable source of urea. In pools with children, frequent bathroom breaks help.
- Watch the landscaped surroundings: if there's lawn or plants right next to the pool, check that watering and drainage aren't carrying fertilized water toward it.
- Renew water periodically: replacing part of the volume with fresh water, something many pools already do naturally through evaporation and topping up, helps "dilute" what has built up over the season.
- Keep filtration and disinfection within range: a filtration system that works well and a correct chlorine level help keep these compounds from turning into a visible problem. You can review the reference ranges in our water parameters guide.
Frequently asked questions
Are nitrates and nitrites dangerous to swimmers' health?
At the concentrations typical of a well-maintained pool, they don't pose a direct health risk. The real problem is indirect: they feed algae growth and cause chlorine to be consumed faster than expected.
Can you measure nitrate levels at home?
Yes, test strips and specific kits for nitrates exist, though they're less common than chlorine and pH tests. In professional maintenance, they're usually checked when chlorine consumption doesn't match what's expected.
Does showering before swimming really make a difference?
Yes, quite a bit. Even a few seconds under the shower with soap removes a good part of the sweat, sunscreen and organic residue on the skin, which is the main ongoing source of urea and nitrogen in the water during swimming.