After my last post on lead in paint, I wanted to give people a more intuitive feel for what PPM actually means. So I made a video. Then I made these.
The basic analogy I use: 1 part per million (PPM) is roughly 1 grain of rice in a 100-pound bag. About a million grains of rice weigh 100 lbs. So if you find one red-dyed grain in 100 lbs of plain white rice, you're looking at 1 PPM red dye contamination. That's the unit.
The video version of this used Red 40 food coloring on actual rice. The static version below uses a 40,000-cell grid — call it a thousand-grain "sample" of a bigger million-grain bag. Each red dot is a contaminated grain. The five panels show what 90, 600, 3,000, 5,000, and 40,000 PPM lead in paint actually look like.
90 PPM — the current legal limit
This is the current legal limit for new consumer paint in the United States, set under 16 CFR Part 1303 and effective since 2009. Pretty clearly safe. Very few "particles" of lead present.
It's worth pausing here: lead is a naturally occurring element. Manufacturing concentrates trace contaminants. Whether lowering the limit further makes practical sense, given how widely lead exists in the environment background, is a real question — and one I'll come back to.
600 PPM — 6× the limit
At 600 PPM, the surface is presenting noticeably more contamination — roughly six times the modern limit. This is well below the federal "lead-based paint" threshold of 5,000 PPM, but there's no health-based justification for that 5,000 line. (Future post.)
3,000 PPM — 33× the limit
At 3,000 PPM the contamination really starts to populate the visual field. This is 33× the safe level. The grid stops looking "mostly clean."
5,000 PPM — the federal "lead-based paint" threshold
This is the federal threshold for "lead-based paint" — about 66× the current legal limit for new paint. Visually striking. And here's the catch: this number isn't health-derived. It is, very closely, the practical lower detection bound of XRF (X-ray fluorescence) instruments measuring through layered paint.
XRF is the principal field-detection method used by lead inspectors and risk assessors. It uses X-ray bombardment to make atoms emit characteristic fluorescent X-rays. The technique works — but it can't resolve individual paint layers. So it averages lead content across everything on the surface, including overpaint. That averaging is why the threshold is high: lower thresholds get noisy because of how the measurement works, not because lower concentrations are safe.
The dust generated by sanding or scraping paint at this concentration is extraordinarily potent. Even when "averaged" with non-lead overpaint dust, it's still a hazard. The federal threshold is a measurement-tool artifact, not a biological one.
40,000 PPM — historic lead paint at full potency
This is what the original lead paints actually were. Roughly 4% lead by weight. 40,000 PPM. This is the paint your great-grandparents put on the trim and that your grandparents painted over with latex. Approximately 444× the modern legal limit.
Every time someone sands one of those layers, the released dust is at this potency.
A teaser: what PPB really means
PPM is parts per million. PPB — parts per billion — is one thousand times finer.
If 1 PPM is roughly one grain of rice in 100 lbs of rice, then 1 PPB is roughly one grain in 100,000 lbs of rice. About a small grain silo's worth.
To visualize 1 PPB on the same 40,000-cell grid above, you'd need 0.00004 of a dot. The image is empty because there is nothing visible to plot. To get one dot to show up at PPB, the grid would need to be roughly 25,000× bigger — a wall of about 31,623 by 31,623 cells.
This matters because PPB is the unit you'll see in modern food safety, drinking water, baby food, and prenatal vitamin standards. When someone says "this baby food contains 5 PPB of lead," they're talking about a quantity that is, geometrically, three orders of magnitude smaller than anything I drew above. Whether that's "a lot" or "a little" depends entirely on how much of the food gets eaten, by whom, and how often. We'll dig into PPB exposure math in a future post.
Key facts
- 1 PPM by weight = 1 mg per kg = 0.0001%; ≈ 1 grain of rice in 100 lbs of rice
- Current U.S. legal limit for new paint: 90 PPM (16 CFR Part 1303, effective 2009)
- Federal "lead-based paint" threshold: 5,000 PPM (0.5%) — drives RRP regulatory triggers
- Historic lead paint potency: ~40,000 PPM (4%)
- 5,000 PPM threshold rationale: closely matches practical XRF detection floor through layered paint, not a health-based number
- PPB vs PPM: 1 PPB = 1/1000 of 1 PPM = 1 microgram per kilogram
Frequently asked questions
What does PPM mean?
Parts per million — 1 part of substance in 1 million parts of the total. By weight, 1 PPM ≈ 1 grain of rice in 100 lbs of rice.
What is the legal limit for lead in paint?
90 PPM in new consumer paint, set by the U.S. CPSC under 16 CFR Part 1303 (effective 2009). The 5,000 PPM "lead-based paint" threshold defines paint that triggers federal RRP renovation requirements.
Why is the 5,000 PPM threshold so much higher than 90 PPM?
It's a tool-driven number, not a health-driven one. 5,000 PPM is roughly the practical lower detection bound of XRF instruments measuring through layered paint, where individual layers can't be resolved.
What is PPB?
Parts per billion — 1,000 times finer than PPM. 1 PPB ≈ 1 grain of rice in 100,000 lbs of rice. Used for food, water, and consumer products at health-relevant low concentrations.
How does XRF detect lead?
An X-ray beam excites atoms in the sample; the atoms re-emit fluorescent X-rays at element-specific wavelengths. The instrument measures lead's characteristic emissions. Non-destructive, but averages across all paint layers.
References
- U.S. CPSC, "Ban of Lead-Containing Paint…," 16 CFR Part 1303 (1977 final rule; 90 PPM threshold effective 2009)
- U.S. EPA, "Lead-Based Paint Renovation, Repair and Painting Rule (RRP)," 40 CFR Part 745 Subpart E
- U.S. EPA, "X-Ray Fluorescence (XRF) Lead Paint Inspection Methodology," HUD/EPA performance characteristic sheets
- U.S. FDA, "Closer to Zero" — action levels for lead in baby foods (PPB framework)