Note that concentrations matter, that there is a dose-response curve (more lead => greater impacts), and that the quantities of lead in common products such as paint were insane.

Lead wasn't simply some trace component of paint, but was up to half by weight (dried), or 500,000 ppm in the US (and probably comparable if not worse elsewhere:

White house paint contained up to 50% lead before 1955. Federal law lowered the amount of lead allowable in paint to 1% in 1971. In 1977, the Consumer Products Safety Commission limited the lead in most paints to 0.06% (600 ppm by dry weight). Since 2009, the lead allowable in most paints is now 0.009%. Paint for bridges and marine use may contain greater amounts of lead.

https://www.atsdr.cdc.gov/csem/leadtoxicity/safety_standards...

Leaded petrol had on the order of 0.5 -- 1 g/l lead, again, not trace amounts, which were discharged directly into the air. Much settled out relatively quickly, within a few hundred metres, as lead is heavy. Still, lead permeated cities and land adjoining roadways and expressways. And still does. Remediation is expensive, natural remediation takes centuries.

At 90 ppm, India's regulations seem to impose a maximum upper bound, and controls for incidental introduction. Given that total elimination is impossible, setting a maximum standard.

The US standard is also 90 ppm:

https://www.cpsc.gov/Business--Manufacturing/Business-Educat...

When I was a teen, copper and tin were too expensive, and I was a bright, precocious teen sculptor. So picture eleven, twelve year old me melting lead in a crucible, taking the crucible in flaking asbestos gloves and pouring it into my casting form. I was a productive little sculptor, too, making many pieces a year.

Partly though, if I'm remembering correctly, is that no amount of lead is considered 'safe'. Obviously that's very difficult, and many people will be exposed to lead at some point. End goal should be to minimize, and make the ppm as near zero as possible.

Again: it's a matter of quantity, background or other source rates, costs of reduction, measurement capability, and alternative harms, etc.

All quality standards are specified on some acceptable deviation. The goal is to not exceed the standard.

Equipment, water supply, source materials, packaging, extant air containation (if in an area in which, say, leaded fuels remain in use), etc., might all contribute to trace contamination. If you have an interest in the reasoning / conditions, rulemaking hearings and evidence likely refer to this.

I'm not about to dive into this, but if you have a genuine interest, the US regulation is in 16 CFR 1303.1, created in 1977, amended in 2008. Hearings reports are likely available and will have scientific, industry, and public-interest statements.

https://www.ecfr.gov/current/title-16/chapter-II/subchapter-...

https://www.cpsc.gov/Regulations-Laws--Standards